Queryable.FirstOrDefault Method

Definition

Namespace:

System.Linq

Assembly:

System.Linq.Queryable.dll

Returns the first element of a sequence, or a default value if no element is found.

Overloads

FirstOrDefault<TSource>(IQueryable<TSource>)	Returns the first element of a sequence, or a default value if the sequence contains no elements.
FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)	Returns the first element of a sequence that satisfies a specified condition or a default value if no such element is found.
FirstOrDefault<TSource>(IQueryable<TSource>, TSource)	Returns the first element of a sequence, or a default value if the sequence contains no elements.
FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>, TSource)	Returns the first element of the sequence that satisfies a condition or a default value if no such element is found.

FirstOrDefault<TSource>(IQueryable<TSource>)

Source:

Queryable.cs

Returns the first element of a sequence, or a default value if the sequence contains no elements.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource FirstOrDefault(System::Linq::IQueryable<TSource> ^ source);

public static TSource? FirstOrDefault<TSource> (this System.Linq.IQueryable<TSource> source);

static member FirstOrDefault : System.Linq.IQueryable<'Source> -> 'Source

<Extension()>
Public Function FirstOrDefault(Of TSource) (source As IQueryable(Of TSource)) As TSource

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IQueryable<TSource>

The IQueryable<T> to return the first element of.

Returns

TSource

default(TSource) if source is empty; otherwise, the first element in source.

Exceptions

ArgumentNullException

source is null.

Examples

The following code example demonstrates how to use FirstOrDefault<TSource>(IQueryable<TSource>) on an empty sequence.

// Create an empty array.
int[] numbers = { };
// Get the first item in the array, or else the
// default value for type int (0).
int first = numbers.AsQueryable().FirstOrDefault();

Console.WriteLine(first);

/*
    This code produces the following output:

    0
*/

' Create an empty array.
Dim numbers() As Integer = {}
' Get the first item in the array, or else the 
' default value for type int, which is 0.
Dim first As Integer = numbers.AsQueryable().FirstOrDefault()

MsgBox(first)

' This code produces the following output:

' 0

Sometimes the value of default(TSource) is not the default value that you want to use if the collection contains no elements. Instead of checking the result for the unwanted default value and then changing it if necessary, you can use the DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) method to specify the default value that you want to use if the collection is empty. Then, call First<TSource>(IQueryable<TSource>) to obtain the first element. The following code example uses both techniques to obtain a default value of 1 if a collection of numeric months is empty. Because the default value for an integer is 0, which does not correspond to any month, the default value must be specified as 1 instead. The first result variable is checked for the unwanted default value after the query is completed. The second result variable is obtained by calling DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) to specify a default value of 1.

List<int> months = new List<int> { };

// Setting the default value to 1 after the query.
int firstMonth1 = months.AsQueryable().FirstOrDefault();
if (firstMonth1 == 0)
{
    firstMonth1 = 1;
}
Console.WriteLine("The value of the firstMonth1 variable is {0}", firstMonth1);

// Setting the default value to 1 by using DefaultIfEmpty() in the query.
int firstMonth2 = months.AsQueryable().DefaultIfEmpty(1).First();
Console.WriteLine("The value of the firstMonth2 variable is {0}", firstMonth2);

/*
 This code produces the following output:

 The value of the firstMonth1 variable is 1
 The value of the firstMonth2 variable is 1
*/

Dim months As New List(Of Integer)(New Integer() {})

' Setting the default value to 1 after the query.
Dim firstMonth1 As Integer = months.AsQueryable().FirstOrDefault()
If firstMonth1 = 0 Then
    firstMonth1 = 1
End If
MsgBox(String.Format("The value of the firstMonth1 variable is {0}", firstMonth1))

' Setting the default value to 1 by using DefaultIfEmpty() in the query.
Dim firstMonth2 As Integer = months.AsQueryable().DefaultIfEmpty(1).First()
MsgBox(String.Format("The value of the firstMonth2 variable is {0}", firstMonth2))

' This code produces the following output:
'
' The value of the firstMonth1 variable is 1
' The value of the firstMonth2 variable is 1

Remarks

The FirstOrDefault<TSource>(IQueryable<TSource>) method generates a MethodCallExpression that represents calling FirstOrDefault<TSource>(IQueryable<TSource>) itself as a constructed generic method. It then passes the MethodCallExpression to the Execute<TResult>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling FirstOrDefault<TSource>(IQueryable<TSource>) depends on the implementation of the type of the source parameter. The expected behavior is that it returns the first element in source, or a default value if source is empty.

The FirstOrDefault method does not provide a way to specify the default value to return if source is empty. If you want to specify a default value other than default(TSource), use the DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) method as described in the Example section.

FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)

Source:

Queryable.cs

Returns the first element of a sequence that satisfies a specified condition or a default value if no such element is found.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource FirstOrDefault(System::Linq::IQueryable<TSource> ^ source, System::Linq::Expressions::Expression<Func<TSource, bool> ^> ^ predicate);

public static TSource? FirstOrDefault<TSource> (this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,bool>> predicate);

static member FirstOrDefault : System.Linq.IQueryable<'Source> * System.Linq.Expressions.Expression<Func<'Source, bool>> -> 'Source

<Extension()>
Public Function FirstOrDefault(Of TSource) (source As IQueryable(Of TSource), predicate As Expression(Of Func(Of TSource, Boolean))) As TSource

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IQueryable<TSource>

An IQueryable<T> to return an element from.

predicate

Expression<Func<TSource,Boolean>>

A function to test each element for a condition.

Returns

TSource

default(TSource) if source is empty or if no element passes the test specified by predicate; otherwise, the first element in source that passes the test specified by predicate.

Exceptions

ArgumentNullException

source or predicate is null.

Examples

The following code example demonstrates how to use FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) by passing in a predicate. In the second query, there is no element in the sequence that satisfies the condition.

string[] names = { "Hartono, Tommy", "Adams, Terry",
                     "Andersen, Henriette Thaulow",
                     "Hedlund, Magnus", "Ito, Shu" };

// Get the first string in the array that is longer
// than 20 characters, or the default value for type
// string (null) if none exists.
string firstLongName =
    names.AsQueryable().FirstOrDefault(name => name.Length > 20);

Console.WriteLine("The first long name is '{0}'.", firstLongName);

// Get the first string in the array that is longer
// than 30 characters, or the default value for type
// string (null) if none exists.
string firstVeryLongName =
    names.AsQueryable().FirstOrDefault(name => name.Length > 30);

Console.WriteLine(
    "There is {0} name that is longer than 30 characters.",
    string.IsNullOrEmpty(firstVeryLongName) ? "NOT a" : "a");

/*
    This code produces the following output:

    The first long name is 'Andersen, Henriette Thaulow'.
    There is NOT a name that is longer than 30 characters.
*/

Dim names() As String = {"Hartono, Tommy", "Adams, Terry", _
                     "Andersen, Henriette Thaulow", _
                     "Hedlund, Magnus", "Ito, Shu"}

' Get the first string in the array that is longer
' than 20 characters, or the default value for type
' string (null) if none exists.
Dim firstLongName As String = _
            names.AsQueryable().FirstOrDefault(Function(name) name.Length > 20)

MsgBox(String.Format("The first long name is '{0}'.", firstLongName))

' Get the first string in the array that is longer
' than 30 characters, or the default value for type
' string (null) if none exists.
Dim firstVeryLongName As String = _
    names.AsQueryable().FirstOrDefault(Function(name) name.Length > 30)

MsgBox(String.Format( _
    "There is {0} name that is longer than 30 characters.", _
    IIf(String.IsNullOrEmpty(firstVeryLongName), "NOT a", "a")))

' This code produces the following output:
'
' The first long name is 'Andersen, Henriette Thaulow'.
' There is NOT a name that is longer than 30 characters.

Remarks

This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T,TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) method generates a MethodCallExpression that represents calling FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) itself as a constructed generic method. It then passes the MethodCallExpression to the Execute<TResult>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) depends on the implementation of the type of the source parameter. The expected behavior is that it returns the first element in source that satisfies the condition in predicate, or a default value if no element satisfies the condition.

FirstOrDefault<TSource>(IQueryable<TSource>, TSource)

Source:

Queryable.cs

Returns the first element of a sequence, or a default value if the sequence contains no elements.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource FirstOrDefault(System::Linq::IQueryable<TSource> ^ source, TSource defaultValue);

public static TSource FirstOrDefault<TSource> (this System.Linq.IQueryable<TSource> source, TSource defaultValue);

static member FirstOrDefault : System.Linq.IQueryable<'Source> * 'Source -> 'Source

<Extension()>
Public Function FirstOrDefault(Of TSource) (source As IQueryable(Of TSource), defaultValue As TSource) As TSource

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IQueryable<TSource>

The IEnumerable<T> to return the first element of.

defaultValue

TSource

The default value to return if the sequence is empty.

Returns

TSource

defaultValue if source is empty; otherwise, the first element in source.

Exceptions

ArgumentNullException

source is null.

FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>, TSource)

Source:

Queryable.cs

Returns the first element of the sequence that satisfies a condition or a default value if no such element is found.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource FirstOrDefault(System::Linq::IQueryable<TSource> ^ source, System::Linq::Expressions::Expression<Func<TSource, bool> ^> ^ predicate, TSource defaultValue);

public static TSource FirstOrDefault<TSource> (this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,bool>> predicate, TSource defaultValue);

static member FirstOrDefault : System.Linq.IQueryable<'Source> * System.Linq.Expressions.Expression<Func<'Source, bool>> * 'Source -> 'Source

<Extension()>
Public Function FirstOrDefault(Of TSource) (source As IQueryable(Of TSource), predicate As Expression(Of Func(Of TSource, Boolean)), defaultValue As TSource) As TSource

Type Parameters

TSource

The type of the elements of source.

Parameters

source

IQueryable<TSource>

An IEnumerable<T> to return an element from.

predicate

Expression<Func<TSource,Boolean>>

A function to test each element for a condition.

defaultValue

TSource

The default value to return if the sequence is empty.

Returns

TSource

defaultValue if source is empty or if no element passes the test specified by predicate; otherwise, the first element in source that passes the test specified by predicate.

Exceptions

ArgumentNullException

source or predicate is null.