Write safe and efficient C# code

C# provides features that enable you to write verifiable safe code with better performance. If you carefully apply these techniques, fewer scenarios require unsafe code. These features make it easier to use references to value types as method arguments and method returns. When done safely, these techniques minimize copying value types. By using value types, you can minimize the number of allocations and garbage collection passes.

One advantage to using value types is that they often avoid heap allocations. The disadvantage is that they're copied by value. This trade-off makes it harder to optimize algorithms that operate on large amounts of data. The language features highlighted in this article provide mechanisms that enable safe efficient code using references to value types. Use these features wisely to minimize both allocations and copy operations.

Some of the guidance in this article refers to coding practices that are always advisable, not only for the performance benefit. Use the readonly keyword when it accurately expresses design intent:

The article also explains some low-level optimizations that are advisable when you've run a profiler and have identified bottlenecks:

These techniques balance two competing goals:

  • Minimize allocations on the heap.

    Variables that are reference types hold a reference to a location in memory and are allocated on the managed heap. Only the reference is copied when a reference type is passed as an argument to a method or returned from a method. Each new object requires a new allocation, and later must be reclaimed. Garbage collection takes time.

  • Minimize the copying of values.

    Variables that are value types directly contain their value, and the value is typically copied when passed to a method or returned from a method. This behavior includes copying the value of this when calling iterators and async instance methods of structs. The copy operation takes time, depending on the size of the type.

This article uses the following example concept of the 3D-point structure to explain its recommendations:

public struct Point3D
    public double X;
    public double Y;
    public double Z;

Different examples use different implementations of this concept.

Declare immutable structs as readonly

Declare a readonly struct to indicate that a type is immutable. The readonly modifier informs the compiler that your intent is to create an immutable type. The compiler enforces that design decision with the following rules:

  • All field members must be read-only.
  • All properties must be read-only, including auto-implemented properties.

These two rules are sufficient to ensure that no member of a readonly struct modifies the state of that struct. The struct is immutable. The Point3D structure could be defined as an immutable struct as shown in the following example:

readonly public struct ReadonlyPoint3D
    public ReadonlyPoint3D(double x, double y, double z)
        this.X = x;
        this.Y = y;
        this.Z = z;

    public double X { get; }
    public double Y { get; }
    public double Z { get; }

Follow this recommendation whenever your design intent is to create an immutable value type. Any performance improvements are an added benefit. The readonly struct keywords clearly express your design intent.

Declare readonly members for mutable structs

When a struct type is mutable, declare members that don't modify state as readonly members.

Consider a different application that needs a 3D point structure, but must support mutability. The following version of the 3D point structure adds the readonly modifier only to those members that don't modify the structure. Follow this example when your design must support modifications to the struct by some members, but you still want the benefits of enforcing readonly on some members:

public struct Point3D
    public Point3D(double x, double y, double z)
        _x = x;
        _y = y;
        _z = z;

    private double _x;
    public double X
        readonly get => _x;
        set => _x = value;

    private double _y;
    public double Y
        readonly get => _y;
        set => _y = value;

    private double _z;
    public double Z
        readonly get => _z;
        set => _z = value;

    public readonly double Distance => Math.Sqrt(X * X + Y * Y + Z * Z);

    public readonly override string ToString() => $"{X}, {Y}, {Z}";

The preceding sample shows many of the locations where you can apply the readonly modifier: methods, properties, and property accessors. If you use auto-implemented properties, the compiler adds the readonly modifier to the get accessor for read-write properties. The compiler adds the readonly modifier to the auto-implemented property declarations for properties with only a get accessor.

Adding the readonly modifier to members that don't mutate state provides two related benefits. First, the compiler enforces your intent. That member can't mutate the struct's state. Second, the compiler won't create defensive copies of in parameters when accessing a readonly member. The compiler can make this optimization safely because it guarantees that the struct is not modified by a readonly member.

Use ref readonly return statements

Use a ref readonly return when both of the following conditions are true:

  • The return value is a struct larger than IntPtr.Size.
  • The storage lifetime is greater than the method returning the value.

You can return values by reference when the value being returned isn't local to the returning method. Returning by reference means that only the reference is copied, not the structure. In the following example, the Origin property can't use a ref return because the value being returned is a local variable:

public Point3D Origin => new Point3D(0,0,0);

However, the following property definition can be returned by reference because the returned value is a static member:

public struct Point3D
    private static Point3D origin = new Point3D(0,0,0);

    // Dangerous! returning a mutable reference to internal storage
    public ref Point3D Origin => ref origin;

    // other members removed for space

You don't want callers modifying the origin, so you should return the value by ref readonly:

public struct Point3D
    private static Point3D origin = new Point3D(0,0,0);

    public static ref readonly Point3D Origin => ref origin;

    // other members removed for space

Returning ref readonly enables you to save copying larger structures and preserve the immutability of your internal data members.

At the call site, callers make the choice to use the Origin property as a ref readonly or as a value:

var originValue = Point3D.Origin;
ref readonly var originReference = ref Point3D.Origin;

The first assignment in the preceding code makes a copy of the Origin constant and assigns that copy. The second assigns a reference. Notice that the readonly modifier must be part of the declaration of the variable. The reference to which it refers can't be modified. Attempts to do so result in a compile-time error.

The readonly modifier is required on the declaration of originReference.

The compiler enforces that the caller can't modify the reference. Attempts to assign the value directly generate a compile-time error. In other cases, the compiler allocates a defensive copy unless it can safely use the readonly reference. Static analysis rules determine if the struct could be modified. The compiler doesn't create a defensive copy when the struct is a readonly struct or the member is a readonly member of the struct. Defensive copies aren't needed to pass the struct as an in argument.

Use the in parameter modifier

The following sections explain what the in modifier does, how to use it, and when to use it for performance optimization:

The out, ref, and in keywords

The in keyword complements the ref and out keywords to pass arguments by reference. The in keyword specifies that the argument is passed by reference, but the called method doesn't modify the value. The in modifier can be applied to any member that takes parameters, such as methods, delegates, lambdas, local functions, indexers, and operators.

With the addition of the in keyword, C# provides a full vocabulary to express your design intent. Value types are copied when passed to a called method when you don't specify any of the following modifiers in the method signature. Each of these modifiers specifies that a variable is passed by reference, avoiding the copy. Each modifier expresses a different intent:

  • out: This method sets the value of the argument used as this parameter.
  • ref: This method may modify the value of the argument used as this parameter.
  • in: This method doesn't modify the value of the argument used as this parameter.

Add the in modifier to pass an argument by reference and declare your design intent to pass arguments by reference to avoid unnecessary copying. You don't intend to modify the object used as that argument.

The in modifier complements out and ref in other ways as well. You can't create overloads of a method that differ only in the presence of in, out, or ref. These new rules extend the same behavior that had always been defined for out and ref parameters. Like the out and ref modifiers, value types aren't boxed because the in modifier is applied. Another feature of in parameters is that you can use literal values or constants for the argument to an in parameter.

The in modifier can also be used with reference types or numeric values. However, the benefits in those cases are minimal, if any.

There are several ways in which the compiler enforces the read-only nature of an in argument. First of all, the called method can't directly assign to an in parameter. It can't directly assign to any field of an in parameter when that value is a struct type. In addition, you can't pass an in parameter to any method using the ref or out modifier. These rules apply to any field of an in parameter, provided the field is a struct type and the parameter is also a struct type. In fact, these rules apply for multiple layers of member access provided the types at all levels of member access are structs. The compiler enforces that struct types passed as in arguments and their struct members are read-only variables when used as arguments to other methods.

Use in parameters for large structs

You can apply the in modifier to any readonly struct parameter, but this practice is likely to improve performance only for value types that are substantially larger than IntPtr.Size. For simple types (such as sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal and bool, and enum types), any potential performance gains are minimal. Some simple types, such as decimal at 16 bytes in size, are larger than either 4-byte or 8-byte references but not by enough to make a measurable difference in performance in most scenarios. And performance may degrade by using pass-by-reference for types smaller than IntPtr.Size.

The following code shows an example of a method that calculates the distance between two points in 3D space.

private static double CalculateDistance(in Point3D point1, in Point3D point2)
    double xDifference = point1.X - point2.X;
    double yDifference = point1.Y - point2.Y;
    double zDifference = point1.Z - point2.Z;

    return Math.Sqrt(xDifference * xDifference + yDifference * yDifference + zDifference * zDifference);

The arguments are two structures that each contain three doubles. A double is 8 bytes, so each argument is 24 bytes. By specifying the in modifier, you pass a 4-byte or 8-byte reference to those arguments, depending on the architecture of the machine. The difference in size is small, but it can add up when your application calls this method in a tight loop using many different values.

However, the impact of any low-level optimizations like using the in modifier should be measured to validate a performance benefit. For example, you might think that using in on a Guid parameter would be beneficial. The Guid type is 16 bytes in size, twice the size of an 8-byte reference. But such a small difference isn't likely to result in a measurable performance benefit unless it's in a method that's in a time critical hot path for your application.

Optional use of in at call site

Unlike a ref or out parameter, you don't need to apply the in modifier at the call site. The following code shows two examples of calling the CalculateDistance method. The first uses two local variables passed by reference. The second includes a temporary variable created as part of the method call.

var distance = CalculateDistance(pt1, pt2);
var fromOrigin = CalculateDistance(pt1, new Point3D());

Omitting the in modifier at the call site informs the compiler that it's allowed to make a copy of the argument for any of the following reasons:

  • There exists an implicit conversion but not an identity conversion from the argument type to the parameter type.
  • The argument is an expression but doesn't have a known storage variable.
  • An overload exists that differs by the presence or absence of in. In that case, the by value overload is a better match.

These rules are useful as you update existing code to use read-only reference arguments. Inside the called method, you can call any instance method that uses by-value parameters. In those instances, a copy of the in parameter is created.

Because the compiler may create a temporary variable for any in parameter, you can also specify default values for any in parameter. The following code specifies the origin (point 0,0,0) as the default value for the second point:

private static double CalculateDistance2(in Point3D point1, in Point3D point2 = default)
    double xDifference = point1.X - point2.X;
    double yDifference = point1.Y - point2.Y;
    double zDifference = point1.Z - point2.Z;

    return Math.Sqrt(xDifference * xDifference + yDifference * yDifference + zDifference * zDifference);

To force the compiler to pass read-only arguments by reference, specify the in modifier on the arguments at the call site, as shown in the following code:

distance = CalculateDistance(in pt1, in pt2);
distance = CalculateDistance(in pt1, new Point3D());
distance = CalculateDistance(pt1, in Point3D.Origin);

This behavior makes it easier to adopt in parameters over time in large codebases where performance gains are possible. You add the in modifier to method signatures first. Then you can add the in modifier at call sites and create readonly struct types to enable the compiler to avoid creating defensive copies of in parameters in more locations.

Avoid defensive copies

Pass a struct as the argument for an in parameter only if it's declared with the readonly modifier or the method accesses only readonly members of the struct. Otherwise, the compiler must create defensive copies in many situations to ensure that arguments are not mutated. Consider the following example that calculates the distance of a 3D point from the origin:

private static double CalculateDistance(in Point3D point1, in Point3D point2)
    double xDifference = point1.X - point2.X;
    double yDifference = point1.Y - point2.Y;
    double zDifference = point1.Z - point2.Z;

    return Math.Sqrt(xDifference * xDifference + yDifference * yDifference + zDifference * zDifference);

The Point3D structure is not a read-only struct. There are six different property access calls in the body of this method. On first examination, you may think these accesses are safe. After all, a get accessor shouldn't modify the state of the object. But there's no language rule that enforces that. It's only a common convention. Any type could implement a get accessor that modified the internal state.

Without some language guarantee, the compiler must create a temporary copy of the argument before calling any member not marked with the readonly modifier. The temporary storage is created on the stack, the values of the argument are copied to the temporary storage, and the value is copied to the stack for each member access as the this argument. In many situations, these copies harm performance enough that pass-by-value is faster than pass-by-read-only-reference when the argument type isn't a readonly struct and the method calls members that aren't marked readonly. If you mark all methods that don't modify the struct state as readonly, the compiler can safely determine that the struct state isn't modified, and a defensive copy is not needed.

If the distance calculation uses the immutable struct, ReadonlyPoint3D, temporary objects aren't needed:

private static double CalculateDistance3(in ReadonlyPoint3D point1, in ReadonlyPoint3D point2 = default)
    double xDifference = point1.X - point2.X;
    double yDifference = point1.Y - point2.Y;
    double zDifference = point1.Z - point2.Z;

    return Math.Sqrt(xDifference * xDifference + yDifference * yDifference + zDifference * zDifference);

The compiler generates more efficient code when you call members of a readonly struct. The this reference, instead of a copy of the receiver, is always an in parameter passed by reference to the member method. This optimization saves copying when you use a readonly struct as an in argument.

Don't pass a nullable value type as an in argument. The Nullable<T> type isn't declared as a read-only struct. That means the compiler must generate defensive copies for any nullable value type argument passed to a method using the in modifier on the parameter declaration.

You can see an example program that demonstrates the performance differences using BenchmarkDotNet in our samples repository on GitHub. It compares passing a mutable struct by value and by reference with passing an immutable struct by value and by reference. The use of the immutable struct and pass by reference is fastest.

Use ref struct types

Use a ref struct or a readonly ref struct, such as Span<T> or ReadOnlySpan<T>, to work with blocks of memory as a sequence of bytes. The memory used by the span is constrained to a single stack frame. This restriction enables the compiler to make several optimizations. The primary motivation for this feature was Span<T> and related structures. You'll achieve performance improvements from these enhancements by using new and updated .NET APIs that make use of the Span<T> type.

Declaring a struct as readonly ref combines the benefits and restrictions of ref struct and readonly struct declarations. The memory used by the readonly span is restricted to a single stack frame, and the memory used by the readonly span can't be modified.

You may have similar requirements working with memory created using stackalloc or when using memory from interop APIs. You can define your own ref struct types for those needs.

Use nint and nuint types

Native-sized integer types are 32-bit integers in a 32-bit process or 64-bit integers in a 64-bit process. Use them for interop scenarios, low-level libraries, and to optimize performance in scenarios where integer math is used extensively.


Using value types minimizes the number of allocation operations:

  • Storage for value types is stack-allocated for local variables and method arguments.
  • Storage for value types that are members of other objects is allocated as part of that object, not as a separate allocation.
  • Storage for value type return values is stack allocated.

Contrast that with reference types in those same situations:

  • Storage for reference types is heap allocated for local variables and method arguments. The reference is stored on the stack.
  • Storage for reference types that are members of other objects are separately allocated on the heap. The containing object stores the reference.
  • Storage for reference type return values is heap allocated. The reference to that storage is stored on the stack.

Minimizing allocations comes with tradeoffs. You copy more memory when the size of the struct is larger than the size of a reference. A reference is typically 64 bits or 32 bits, and depends on the target machine CPU.

These tradeoffs generally have minimal performance impact. However, for large structs or larger collections, the performance impact increases. The impact can be large in tight loops and hot paths for programs.

These enhancements to the C# language are designed for performance critical algorithms where minimizing memory allocations is a major factor in achieving the necessary performance. You may find that you don't often use these features in the code you write. However, these enhancements have been adopted throughout .NET. As more APIs make use of these features, you'll see the performance of your applications improve.

See also