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Annex C Standard library

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C.1 General

A conforming C# implementation shall provide a minimum set of types having specific semantics. These types and their members are listed here, in alphabetical order by namespace and type. For a formal definition of these types and their members, refer to ISO/IEC 23271:2012 Common Language Infrastructure (CLI), Partition IV; Base Class Library (BCL), Extended Numerics Library, and Extended Array Library, which are included by reference in this specification.

This text is informative.

The standard library is intended to be the minimum set of types and members required by a conforming C# implementation. As such, it contains only those members that are explicitly required by the C# language specification.

It is expected that a conforming C# implementation will supply a significantly more extensive library that enables useful programs to be written. For example, a conforming implementation might extend this library by

  • Adding namespaces.
  • Adding types.
  • Adding members to non-interface types.
  • Adding intervening base classes or interfaces.
  • Having struct and class types implement additional interfaces.
  • Adding attributes (other than the ConditionalAttribute) to existing types and members.

End of informative text.

C.2 Standard Library Types defined in ISO/IEC 23271

Note: Some struct types below have the readonly modifier. This modifier was not available when ISO/IEC 23271 was released, but is required for conforming implementations of this specification. end note

namespace System
{
    public delegate void Action();

    public class ArgumentException : SystemException
    {
        public ArgumentException();
        public ArgumentException(string message);
        public ArgumentException(string message, Exception innerException);
    }

    public class ArithmeticException : Exception
    {
        public ArithmeticException();
        public ArithmeticException(string message);
        public ArithmeticException(string message, Exception innerException);
    }

    public abstract class Array : IList, ICollection, IEnumerable
    {
        public int Length { get; }
        public int Rank { get; }
        public int GetLength(int dimension);
    }

    public class ArrayTypeMismatchException : Exception
    {
        public ArrayTypeMismatchException();
        public ArrayTypeMismatchException(string message);
        public ArrayTypeMismatchException(string message,
            Exception innerException);
    }

    [AttributeUsageAttribute(AttributeTargets.All, Inherited = true,
        AllowMultiple = false)]
    public abstract class Attribute
    {
        protected Attribute();
    }

    public enum AttributeTargets
    {
        Assembly = 0x1,
        Module = 0x2,
        Class = 0x4,
        Struct = 0x8,
        Enum = 0x10,
        Constructor = 0x20,
        Method = 0x40,
        Property = 0x80,
        Field = 0x100,
        Event = 0x200,
        Interface = 0x400,
        Parameter = 0x800,
        Delegate = 0x1000,
        ReturnValue = 0x2000,
        GenericParameter = 0x4000,
        All = 0x7FFF
    }

    [AttributeUsageAttribute(AttributeTargets.Class, Inherited = true)]
    public sealed class AttributeUsageAttribute : Attribute
    {
        public AttributeUsageAttribute(AttributeTargets validOn);
        public bool AllowMultiple { get; set; }
        public bool Inherited { get; set; }
        public AttributeTargets ValidOn { get; }
    }

    public readonly struct Boolean { }
    public readonly struct Byte { }
    public readonly struct Char { }
    public readonly struct Decimal { }
    public abstract class Delegate { }

    public class DivideByZeroException : ArithmeticException
    {
        public DivideByZeroException();
        public DivideByZeroException(string message);
        public DivideByZeroException(string message, Exception innerException);
    }

    public readonly struct Double { }

    public abstract class Enum : ValueType
    {
        protected Enum();
    }

    public class Exception
    {
        public Exception();
        public Exception(string message);
        public Exception(string message, Exception innerException);
        public sealed Exception InnerException { get; }
        public virtual string Message { get; }
    }

    public class GC { }

    public interface IDisposable
    {
        void Dispose();
    }

    public interface IFormattable { }

    public sealed class IndexOutOfRangeException : Exception
    {
        public IndexOutOfRangeException();
        public IndexOutOfRangeException(string message);
        public IndexOutOfRangeException(string message,
            Exception innerException);
    }

    public readonly struct Int16 { }
    public readonly struct Int32 { }
    public readonly struct Int64 { }
    public readonly struct IntPtr { }

    public class InvalidCastException : Exception
    {
        public InvalidCastException();
        public InvalidCastException(string message);
        public InvalidCastException(string message, Exception innerException);
    }

    public class InvalidOperationException : Exception
    {
        public InvalidOperationException();
        public InvalidOperationException(string message);
        public InvalidOperationException(string message,
            Exception innerException);
    }

    public class NotSupportedException : Exception
    {
        public NotSupportedException();
        public NotSupportedException(string message);
        public NotSupportedException(string message, 
            Exception innerException);    
    }

    public struct Nullable<T>
    {
        public bool HasValue { get; }
        public T Value { get; }
    }

    public class NullReferenceException : Exception
    {
        public NullReferenceException();
        public NullReferenceException(string message);
        public NullReferenceException(string message, Exception innerException);
    }

    public class Object
    {
        public Object();
        ~Object();
        public virtual bool Equals(object obj);
        public virtual int GetHashCode();
        public Type GetType();
        public virtual string ToString();
    }

    [AttributeUsageAttribute(AttributeTargets.Class | AttributeTargets.Struct |
        AttributeTargets.Enum | AttributeTargets.Interface |
        AttributeTargets.Constructor | AttributeTargets.Method |
        AttributeTargets.Property | AttributeTargets.Field |
        AttributeTargets.Event | AttributeTargets.Delegate, Inherited = false)]
    public sealed class ObsoleteAttribute : Attribute
    {
        public ObsoleteAttribute();
        public ObsoleteAttribute(string message);
        public ObsoleteAttribute(string message, bool error);
        public bool IsError { get; }
        public string Message { get; }
    }

    public class OutOfMemoryException : Exception
    {
        public OutOfMemoryException();
        public OutOfMemoryException(string message);
        public OutOfMemoryException(string message, Exception innerException);
    }

    public class OverflowException : ArithmeticException
    {
        public OverflowException();
        public OverflowException(string message);
        public OverflowException(string message, Exception innerException);
    }

    public readonly struct SByte { }
    public readonly struct Single { }

    public sealed class StackOverflowException : Exception
    {
        public StackOverflowException();
        public StackOverflowException(string message);
        public StackOverflowException(string message, Exception innerException);
    }

    public sealed class String : IEnumerable<Char>, IEnumerable
    {
        public int Length { get; }
        public char this [int index] { get; }
        public static string Format(string format, params object[] args);
    }

    public abstract class Type : MemberInfo { }

    public sealed class TypeInitializationException : Exception
    {
        public TypeInitializationException(string fullTypeName,
            Exception innerException);
    }

    public readonly struct UInt16 { }
    public readonly struct UInt32 { }
    public readonly struct UInt64 { }
    public readonly struct UIntPtr { }

    public abstract class ValueType
    {
        protected ValueType();
    }
}

namespace System.Collections
{
    public interface ICollection : IEnumerable
    {
        int Count { get; }
        bool IsSynchronized { get; }
        object SyncRoot { get; }
        void CopyTo(Array array, int index);
    }

    public interface IEnumerable
    {
        IEnumerator GetEnumerator();
    }

    public interface IEnumerator
    {
        object Current { get; }
        bool MoveNext();
        void Reset();
    }

    public interface IList : ICollection, IEnumerable
    {
        bool IsFixedSize { get; }
        bool IsReadOnly { get; }
        object this [int index] { get; set; }
        int Add(object value);
        void Clear();
        bool Contains(object value);
        int IndexOf(object value);
        void Insert(int index, object value);
        void Remove(object value);
        void RemoveAt(int index);
    }
}

namespace System.Collections.Generic
{
    public interface ICollection<T> : IEnumerable<T>
    {
        int Count { get; }
        bool IsReadOnly { get; }
        void Add(T item);
        void Clear();
        bool Contains(T item);
        void CopyTo(T[] array, int arrayIndex);
        bool Remove(T item);
    }

    public interface IEnumerable<T> : IEnumerable
    {
        IEnumerator<T> GetEnumerator();
    }

    public interface IEnumerator<T> : IDisposable, IEnumerator
    {
        T Current { get; }
    }

    public interface IList<T> : ICollection<T>
    {
        T this [int index] { get; set; }
        int IndexOf(T item);
        void Insert(int index, T item);
        void RemoveAt(int index);
    }
}

namespace System.Diagnostics
{
    [AttributeUsageAttribute(AttributeTargets.Method | AttributeTargets.Class,
                             AllowMultiple = true)]
    public sealed class ConditionalAttribute : Attribute
    {
        public ConditionalAttribute(string conditionString);
        public string ConditionString { get; }
    }
}

namespace System.Reflection
{
    public abstract class MemberInfo
    {
        protected MemberInfo();
    }
}

namespace System.Runtime.CompilerServices
{
    public sealed class IndexerNameAttribute : Attribute
    {
        public IndexerNameAttribute(String indexerName);
    }

    public static class Unsafe
    {
        public static ref T NullRef<T>();
    }
}

namespace System.Threading
{
    public static class Monitor
    {
        public static void Enter(object obj);
        public static void Exit(object obj);
    }
}

C.3 Standard Library Types not defined in ISO/IEC 23271

The following types, including the members listed, shall be defined in a conforming standard library. (These types might be defined in a future edition of ISO/IEC 23271.) It is expected that many of these types will have more members available than are listed.

A conforming implementation may provide Task.GetAwaiter() and Task<TResult>.GetAwaiter() as extension methods.

namespace System
{
    public class FormattableString : IFormattable { }

    public class OperationCanceledException : Exception
    {
        public OperationCanceledException();
        public OperationCanceledException(string message);
        public OperationCanceledException(string message, Exception innerException);
    }

    public readonly ref struct ReadOnlySpan<T>
    {
        public int Length { get; }
        public ref readonly T this[int index] { get; }
    }

    public readonly ref struct Span<T>
    {
        public int Length { get; }
        public ref T this[int index] { get; }
        public static implicit operator ReadOnlySpan<T>(Span<T> span);
    }

    public struct ValueTuple<T1>
    {
        public T1 Item1;
        public ValueTuple(T1 item1);
    }

    public struct ValueTuple<T1, T2>
    {
        public T1 Item1;
        public T2 Item2;
        public ValueTuple(T1 item1, T2 item2);
    }

    public struct ValueTuple<T1, T2, T3>
    {
        public T1 Item1;
        public T2 Item2;
        public T3 Item3;
        public ValueTuple(T1 item1, T2 item2, T3 item3);
    }

    public struct ValueTuple<T1, T2, T3, T4>
    {
        public T1 Item1;
        public T2 Item2;
        public T3 Item3;
        public T4 Item4;
        public ValueTuple(T1 item1, T2 item2, T3 item3, T4 item4);
    }

    public struct ValueTuple<T1, T2, T3, T4, T5>
    {
        public T1 Item1;
        public T2 Item2;
        public T3 Item3;
        public T4 Item4;
        public T5 Item5;
        public ValueTuple(T1 item1, T2 item2, T3 item3, T4 item4, T5 item5);
    }

    public struct ValueTuple<T1, T2, T3, T4, T5, T6>
    {
        public T1 Item1;
        public T2 Item2;
        public T3 Item3;
        public T4 Item4;
        public T5 Item5;
        public T6 Item6;
        public ValueTuple(T1 item1, T2 item2, T3 item3, T4 item4, T5 item5,
            T6 item6);
    }

    public struct ValueTuple<T1, T2, T3, T4, T5, T6, T7>
    {
        public T1 Item1;
        public T2 Item2;
        public T3 Item3;
        public T4 Item4;
        public T5 Item5;
        public T6 Item6;
        public T7 Item7;
        public ValueTuple(T1 item1, T2 item2, T3 item3, T4 item4, T5 item5,
            T6 item6, T7 item7);
    }

    public struct ValueTuple<T1, T2, T3, T4, T5, T6, T7, TRest>
    {
        public T1 Item1;
        public T2 Item2;
        public T3 Item3;
        public T4 Item4;
        public T5 Item5;
        public T6 Item6;
        public T7 Item7;
        public TRest Rest;
        public ValueTuple(T1 item1, T2 item2, T3 item3, T4 item4, T5 item5,
            T6 item6, T7 item7, TRest rest);
    }
}

namespace System.Collections.Generic
{
    public interface IReadOnlyCollection<out T> : IEnumerable<T>
    {
        int Count { get; }
    }

    public interface IReadOnlyList<out T> : IReadOnlyCollection<T>
    {
        T this [int index] { get; }
    }
}

namespace System.Linq.Expressions
{
    public sealed class Expression<TDelegate>
    {
        public TDelegate Compile();
    }
}

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | 
        AttributeTargets.Interface, 
        Inherited = false, AllowMultiple = false)]
    public sealed class AsyncMethodBuilderAttribute : Attribute
    {
        public AsyncMethodBuilderAttribute(Type builderType) {}
 
        public Type BuilderType { get; }
    }

    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
    public sealed class CallerFilePathAttribute : Attribute
    {
        public CallerFilePathAttribute() { }
    }

    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
    public sealed class CallerLineNumberAttribute : Attribute
    {
        public CallerLineNumberAttribute() { }
    }

    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
    public sealed class CallerMemberNameAttribute : Attribute
    {
        public CallerMemberNameAttribute() { }
    }

    public static class FormattableStringFactory
    {
        public static FormattableString Create(string format,
            params object[] arguments);
    }

    public interface ICriticalNotifyCompletion : INotifyCompletion
    {
        void UnsafeOnCompleted(Action continuation);
    }

    public interface INotifyCompletion
    {
        void OnCompleted(Action continuation);
    }

    public readonly struct TaskAwaiter : ICriticalNotifyCompletion,
        INotifyCompletion
    {
        public bool IsCompleted { get; }
        public void GetResult();
    }

    public readonly struct TaskAwaiter<TResult> : ICriticalNotifyCompletion,
        INotifyCompletion
    {
        public bool IsCompleted { get; }
        public TResult GetResult();
    }

    public readonly struct ValueTaskAwaiter : ICriticalNotifyCompletion,
        INotifyCompletion
    {
        public bool IsCompleted { get; }
        public void GetResult();
    }

    public readonly struct ValueTaskAwaiter<TResult>
        : ICriticalNotifyCompletion, INotifyCompletion
    {
        public bool IsCompleted { get; }
        public TResult GetResult();
    }
}

namespace System.Threading.Tasks
{
    public class Task
    {
        public System.Runtime.CompilerServices.TaskAwaiter GetAwaiter();
    }

    public class Task<TResult> : Task
    {
        public new System.Runtime.CompilerServices.TaskAwaiter<T> GetAwaiter();
    }

    public readonly struct ValueTask : System.IEquatable<ValueTask>
    {
        public System.Runtime.CompilerServices.ValueTaskAwaiter GetAwaiter();
    }

    public readonly struct ValueTask<TResult>
        : System.IEquatable<ValueTask<TResult>>
    {
        public new System.Runtime.CompilerServices.ValueTaskAwaiter<TResult>
            GetAwaiter();
    }
}

C.4 Format Specifications

The meaning of the formats, as used in interpolated string expressions (§12.8.3), are defined in ISO/IEC 23271:2012. For convenience the following text is copied from the description of System.IFormattable.

This text is informative.

A format is a string that describes the appearance of an object when it is converted to a string. Either standard or custom formats can be used. A standard format takes the form Axx, where A is a single alphabetic character called the format specifier, and xx is an integer between zero and 99 inclusive, called the precision specifier. The format specifier controls the type of formatting applied to the value being represented as a string. The precision specifier controls the number of significant digits or decimal places in the string, if applicable.

Note: For the list of standard format specifiers, see the table below. Note that a given data type, such as System.Int32, might not support one or more of the standard format specifiers. end note

Note: When a format includes symbols that vary by culture, such as the currencysymbol included by the ‘C’ and ‘c’ formats, a formatting object supplies the actual characters used in the string representation. A method might include a parameter to pass a System.IFormatProvider object that supplies a formatting object, or the method might use the default formatting object, which contains the symbol definitions for the current culture. The current culture typically uses the same set of symbols used system-wide by default. In the Base Class Library, the formatting object for system-supplied numeric types is a System.Globalization.NumberFormatInfo instance. For System.DateTime instances, a System.Globalization.DateTimeFormatInfo is used. end note

The following table describes the standard format specifiers and associated formatting object members that are used with numeric data types in the Base Class Library.

Format Specifier Description

C

c

Currency Format: Used for strings containing a monetary value. The System.Globalization.NumberFormatInfo.CurrencySymbol, System.Globalization.NumberFormatInfo.CurrencyGroupSizes, System.Globalization.NumberFormatInfo.CurrencyGroupSeparator, and System.Globalization.NumberFormatInfo.CurrencyDecimalSeparator members of a System.Globalization.NumberFormatInfo supply the currency symbol, size and separator for digit groupings, and decimal separator, respectively.

System.Globalization.NumberFormatInfo.CurrencyNegativePattern and System.Globalization.NumberFormatInfo.CurrencyPositivePattern determine the symbols used to represent negative and positive values. For example, a negative value can be prefixed with a minus sign, or enclosed in parentheses.

If the precision specifier is omitted, System.Globalization.NumberFormatInfo.CurrencyDecimalDigits determines the number of decimal places in the string. Results are rounded to the nearest representable value when necessary.

D

d

Decimal Format: (This format is valid only when specified with integral data types.) Used for strings containing integer values. Negative numbers are prefixed with the negative number symbol specified by the System.Globalization.NumberFormatInfo.NegativeSign property.

The precision specifier determines the minimum number of digits that appear in the string. If the specified precision requires more digits than the value contains, the string is left-padded with zeros. If the precision specifier specifies fewer digits than are in the value, the precision specifier is ignored.

E

e

Scientific (Engineering) Format: Used for strings in one of the following forms:

          [-]m.ddddddE+xxx

          [-]m.ddddddE-xxx

          [-]m.dddddde+xxx

          [-]m.dddddde-xxx

The negative number symbol (‘-’) appears only if the value is negative, and is supplied by the System.Globalization.NumberFormatInfo.NegativeSign property.

Exactly one non-zero decimal digit (m) precedes the decimal separator (‘.’), which is supplied by the System.Globalization.NumberFormatInfo.NumberDecimalSeparator property.

The precision specifier determines the number of decimal places (dddddd) in the string. If the precision specifier is omitted, six decimal places are included in the string.

The exponent (+/-xxx) consists of either a positive or negative number symbol followed by a minimum of three digits (xxx). The exponent is left-padded with zeros, if necessary. The case of the format specifier (‘E’ or ‘e’) determines the case used for the exponent prefix (E or e) in the string. Results are rounded to the nearest representable value when necessary. The positive number symbol is supplied by the System.Globalization.NumberFormatInfo.PositiveSign property.

F

f

Fixed-Point Format: Used for strings in the following form:

          [-]m.dd...d

At least one non-zero decimal digit (m) precedes the decimal separator (‘.’), which is supplied by the System.Globalization.NumberFormatInfo.NumberDecimalSeparator property.

A negative number symbol sign (‘-’) precedes m only if the value is negative. This symbol is supplied by the System.Globalization.NumberFormatInfo.NegativeSign property.

The precision specifier determines the number of decimal places (dd...d) in the string. If the precision specifier is omitted, System.Globalization.NumberFormatInfo.NumberDecimalDigits determines the number of decimal places in the string. Results are rounded to the nearest representable value when necessary.

G

g

General Format: The string is formatted in either fixed-point format (‘F’ or ‘f’) or scientific format (‘E’ or ‘e’).

For integral types:

Values are formatted using fixed-point format if exponent < precision specifier, where exponent is the exponent of the value in scientific format. For all other values, scientific format is used.

If the precision specifier is omitted, a default precision equal to the field width required to display the maximum value for the data type is used, which results in the value being formatted in fixed-point format. The default precisions for integral types are as follows:

          System.Int16, System.UInt16 : 5

          System.Int32, System.UInt32 : 10

          System.Int64, System.UInt64 : 19

For Single, Decimal and Double types:

Values are formatted using fixed-point format if exponent ≥ -4 and exponent < precision specifier, where exponent is the exponent of the value in scientific format. For all other values, scientific format is used. Results are rounded to the nearest representable value when necessary.

If the precision specifier is omitted, the following default precisions are used:

          System.Single : 7

          System.Double : 15

          System.Decimal : 29

For all types:

  • The number of digits that appear in the result (not including the exponent) will not exceed the value of the precision specifier; values are rounded as necessary.
  • The decimal point and any trailing zeros after the decimal point are removed whenever possible.
  • The case of the format specifier (‘G’ or ‘g’) determines whether ‘E’ or ‘e’ prefixes the scientific format exponent.

N

n

Number Format: Used for strings in the following form:

          [-]d,ddd,ddd.dd...d

The representation of negative values is determined by the System.Globalization.NumberFormatInfo.NumberNegativePattern property. If the pattern includes a negative number symbol (‘-’), this symbol is supplied by the System.Globalization.NumberFormatInfo.NegativeSign property.

At least one non-zero decimal digit (d) precedes the decimal separator (‘.’), which is supplied by the System.Globalization.NumberFormatInfo.NumberDecimalSeparator property. Digits between the decimal point and the most significant digit in the value are grouped using the group size specified by the System.Globalization.NumberFormatInfo.NumberGroupSizes property. The group separator (‘,’) is inserted between each digit group, and is supplied by the System.Globalization.NumberFormatInfo.NumberGroupSeparator property.

The precision specifier determines the number of decimal places (dd...d). If the precision specifier is omitted, System.Globalization.NumberFormatInfo.NumberDecimalDigits determines the number of decimal places in the string. Results are rounded to the nearest representable value when necessary.

P

p

Percent Format: Used for strings containing a percentage. The System.Globalization.NumberFormatInfo.PercentSymbol, System.Globalization.NumberFormatInfo.PercentGroupSizes, System.Globalization.NumberFormatInfo.PercentGroupSeparator, and System.Globalization.NumberFormatInfo.PercentDecimalSeparator members of a System.Globalization.NumberFormatInfo supply the percent symbol, size and separator for digit groupings, and decimal separator, respectively.

System.Globalization.NumberFormatInfo.PercentNegativePattern and System.Globalization.NumberFormatInfo.PercentPositivePattern determine the symbols used to represent negative and positive values. For example, a negative value can be prefixed with a minus sign, or enclosed in parentheses.

If no precision is specified, the number of decimal places in the result is determined by System.Globalization.NumberFormatInfo.PercentDecimalDigits. Results are rounded to the nearest representable value when necessary.

The result is scaled by 100 (.99 becomes 99%).

R

r

 Round trip Format: (This format is valid only when specified with System.Double or System.Single.) Used to ensure that the precision of the string representation of a floating-point value is such that parsing the string does not result in a loss of precision when compared to the original value. If the maximum precision of the data type (7 for System.Single, and 15 for System.Double) would result in a loss of precision, the precision is increased by two decimal places. If a precision specifier is supplied with this format specifier, it is ignored. This format is otherwise identical to the fixed-point format.

X

x

 Hexadecimal Format: (This format is valid only when specified with integral data types.) Used for string representations of numbers in Base 16. The precision determines the minimum number of digits in the string. If the precision specifies more digits than the number contains, the number is left-padded with zeros. The case of the format specifier (‘X’ or ‘x’) determines whether upper case or lower case letters are used in the hexadecimal representation.

If the numerical value is a System.Single or System.Double with a value of NaN, PositiveInfinity, or NegativeInfinity, the format specifier is ignored, and one of the following is returned: System.Globalization.NumberFormatInfo.NaNSymbol, System.Globalization.NumberFormatInfo.PositiveInfinitySymbol, or System.Globalization.NumberFormatInfo.NegativeInfinitySymbol.

A custom format is any string specified as a format that is not in the form of a standard format string (Axx) described above. The following table describes the characters that are used in constructing custom formats.

Format Specifier Description
0 (zero)

Zero placeholder: If the value being formatted has a digit in the position where a ‘0’ appears in the custom format, then that digit is copied to the output string; otherwise a zero is stored in that position in the output string. The position of the leftmost ‘0’ before the decimal separator and the rightmost ‘0’ after the decimal separator determine the range of digits that are always present in the output string.

The number of Zero and/or Digit placeholders after the decimal separator determines the number of digits that appear after the decimal separator. Values are rounded as necessary.
#

Digit placeholder: If the value being formatted has a digit in the position where a ‘#’ appears in the custom format, then that digit is copied to the output string; otherwise, nothing is stored in that position in the output string. Note that this specifier never stores the ‘0’ character if it is not a significant digit, even if ‘0’ is the only digit in the string. (It does display the ‘0’ character in the output string if it is a significant digit.)

The number of Zero and/or Digit placeholders after the decimal separator determines the number of digits that appear after the decimal separator. Values are rounded as necessary.
. (period) Decimal separator: The left most ‘.’ character in the format string determines the location of the decimal separator in the formatted value; any additional ‘.’ characters are ignored. The System.Globalization.NumberFormatInfo.NumberDecimalSeparator property determines the symbol used as the decimal separator.
, (comma)

Group separator and number scaling: The ‘,’ character serves two purposes. First, if the custom format contains this character between two Zero or Digit placeholders (0 or #) and to the left of the decimal separator if one is present, then the output will have group separators inserted between each group of digits to the left of the decimal separator. The System.Globalization.NumberFormatInfo.NumberGroupSeparator and System.Globalization.NumberFormatInfo.NumberGroupSizes properties determine the symbol used as the group separator and the number of digits in each group, respectively.

If the format string contains one or more ‘,’ characters immediately to the left of the decimal separator, then the number will be scaled. The scale factor is determined by the number of group separator characters immediately to the left of the decimal separator. If there are x characters, then the value is divided by 1000X before it is formatted. For example, the format string ‘0,,’ will divide a value by one million. Note that the presence of the ‘,’ character to indicate scaling does not insert group separators in the output string. Thus, to scale a number by 1 million and insert group separators, use a custom format similar to ‘#,##0,,’.
% (percent) Percentage placeholder: The presence of a ‘%’ character in a custom format causes a number to be multiplied by 100 before it is formatted. The percent symbol is inserted in the output string at the location where the ‘%’ appears in the format string. The System.Globalization.NumberFormatInfo.PercentSymbol property determines the percent symbol.

E0

E+0

E-0

e0

e+0

e-0

 Engineering format: If any of the strings ‘E’, ‘E+’, ‘E-’, ‘e’, ‘e+’, or ‘e-’ are present in a custom format and is followed immediately by at least one ‘0’ character, then the value is formatted using scientific notation. The number of ‘0’ characters following the exponent prefix (E or e) determines the minimum number of digits in the exponent. The ‘E+’ and ‘e+’ formats indicate that a positive or negative number symbol always precedes the exponent. The ‘E’, ‘E-’, ‘e’, or ‘e-’ formats indicate that a negative number symbol precedes negative exponents; no symbol is precedes positive exponents. The positive number symbol is supplied by the System.Globalization.NumberFormatInfo.PositiveSign property. The negative number symbol is supplied by the System.Globalization.NumberFormatInfo.NegativeSign property.
\ (backslash) Escape character: In some languages, such as C#, the backslash character causes the next character in the custom format to be interpreted as an escape sequence. It is used with C language formatting sequences, such as ‘\n’ (newline). In some languages, the escape character itself is required to be preceded by an escape character when used as a literal. Otherwise, the compiler interprets the character as an escape sequence. This escape character is not required to be supported in all programming languages.

'ABC'

"ABC"

 Literal string: Characters enclosed in single or double quotes are copied to the output string literally, and do not affect formatting.
; (semicolon) Section separator: The ‘;’ character is used to separate sections for positive, negative, and zero numbers in the format string. (This feature is described in detail below.)
Other All other characters: All other characters are stored in the output string as literals in the position in which they appear.

Note that for fixed-point format strings (strings not containing an ‘E0’, ‘E+0’, ‘E-0’, ‘e0’, ‘e+0’, or ‘e-0’), numbers are rounded to as many decimal places as there are Zero or Digit placeholders to the right of the decimal separator. If the custom format does not contain a decimal separator, the number is rounded to the nearest integer. If the number has more digits than there are Zero or Digit placeholders to the left of the decimal separator, the extra digits are copied to the output string immediately before the first Zero or Digit placeholder.

A custom format can contain up to three sections separated by section separator characters, to specify different formatting for positive, negative, and zero values. The sections are interpreted as follows:

  • One section: The custom format applies to all values (positive, negative and zero). Negative values include a negative sign.

  • Two sections: The first section applies to positive values and zeros, and the second section applies to negative values. If the value to be formatted is negative, but becomes zero after rounding according to the format in the second section, then the resulting zero is formatted according to the first section. Negative values do not include a negative sign to allow full control over representations of negative values. For example, a negative can be represented in parenthesis using a custom format similar to ‘####.####;(####.####)’.

  • Three sections: The first section applies to positive values, the second section applies to negative values, and the third section applies to zeros. The second section can be empty (nothing appears between the semicolons), in which case the first section applies to all nonzero values, and negative values include a negative sign. If the number to be formatted is nonzero, but becomes zero after rounding according to the format in the first or second section, then the resulting zero is formatted according to the third section.

The System.Enum and System.DateTime types also support using format specifiers to format string representations of values. The meaning of a specific format specifier varies according to the kind of data (numeric, date/time, enumeration) being formatted. See System.Enum and System.Globalization.DateTimeFormatInfo for a comprehensive list of the format specifiers supported by each type.

C.5 Library Type Abbreviations

The following library types are referenced in this specification. The full names of those types, including the global namespace qualifier are listed below. Throughout this specification, these types appear as either the fully qualified name; with the global namespace qualifier omitted; or as a simple unqualified type name, with the namespace omitted as well. For example, the type ICollection<T>, when used in this specification, always means the type global::System.Collections.Generic.ICollection<T>.

  • global::System.Action
  • global::System.ArgumentException
  • global::System.ArithmeticException
  • global::System.Array
  • global::System.ArrayTypeMisMatchException
  • global::System.Attribute
  • global::System.AttributeTargets
  • global::System.AttributeUsageAttribute
  • global::System.Boolean
  • global::System.Byte
  • global::System.Char
  • global::System.Collections.Generic.ICollection<T>
  • global::System.Collections.Generic.IEnumerable<T>
  • global::System.Collections.Generic.IEnumerator<T>
  • global::System.Collections.Generic.IList<T>
  • global::System.Collections.Generic.IReadonlyCollection<out T>
  • global::System.Collections.Generic.IReadOnlyList<out T>
  • global::System.Collections.ICollection
  • global::System.Collections.IEnumerable
  • global::System.Collections.IList
  • global::System.Collections.IEnumerator
  • global::System.Decimal
  • global::System.Delegate
  • global::System.Diagnostics.ConditionalAttribute
  • global::System.DivideByZeroException
  • global::System.Double
  • global::System.Enum
  • global::System.Exception
  • global::System.GC
  • global::System.ICollection
  • global::System.IDisposable
  • global::System.IEnumerable
  • global::System.IEnumerable<out T>
  • global::System.IList
  • global::System.IndexOutOfRangeException
  • global::System.Int16
  • global::System.Int32
  • global::System.Int64
  • global::System.IntPtr
  • global::System.InvalidCastException
  • global::System.InvalidOperationException
  • global::System.Linq.Expressions.Expression<TDelegate>
  • global::System.MemberInfo
  • global::System.NotSupportedException
  • global::System.Nullable<T>
  • global::System.NullReferenceException
  • global::System.Object
  • global::System.ObsoleteAttribute
  • global::System.OutOfMemoryException
  • global::System.OverflowException
  • global::System.Runtime.CompilerServices.CallerFileAttribute
  • global::System.Runtime.CompilerServices.CallerLineNumberAttribute
  • global::System.Runtime.CompilerServices.CallerMemberNameAttribute
  • global::System.Runtime.CompilerServices.ICriticalNotifyCompletion
  • global::System.Runtime.CompilerServices.IndexerNameAttribute
  • global::System.Runtime.CompilerServices.INotifyCompletion
  • global::System.Runtime.CompilerServices.TaskAwaiter
  • global::System.Runtime.CompilerServices.TaskAwaiter<T>
  • global::System.SByte
  • global::System.Single
  • global::System.StackOverflowException
  • global::System.String
  • global::System.SystemException
  • global::System.Threading.Monitor
  • global::System.Threading.Tasks.Task
  • global::System.Threading.Tasks.Task<TResult>
  • global::System.Type
  • global::System.TypeInitializationException
  • global::System.UInt16
  • global::System.UInt32
  • global::System.UInt64
  • global::System.UIntPtr
  • global::System.ValueType

End of informative text.