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Queryable.Single<TSource> Method (IQueryable<TSource>, Expression<Func<TSource, Boolean>>)

Returns the only element of a sequence that satisfies a specified condition, and throws an exception if more than one such element exists.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)

public static TSource Single<TSource>(
	this IQueryable<TSource> source,
	Expression<Func<TSource, bool>> predicate
)

Type Parameters

TSource

The type of the elements of source.

Parameters

source
Type: System.Linq.IQueryable<TSource>
An IQueryable<T> to return a single element from.
predicate
Type: System.Linq.Expressions.Expression<Func<TSource, Boolean>>
A function to test an element for a condition.

Return Value

Type: TSource
The single element of the input sequence that satisfies the condition in predicate.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable<TSource>. When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

ExceptionCondition
ArgumentNullException

source or predicate is null.

InvalidOperationException

No element satisfies the condition in predicate.

-or-

More than one element satisfies the condition in predicate.

-or-

The source sequence is empty.

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 Single<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) method generates a MethodCallExpression that represents calling Single<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 Single<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 only element in source that satisfies the condition specified by predicate.

The following code example demonstrates how to use Single<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) to select the only element of an array that satisfies a condition.


            string[] fruits = { "apple", "banana", "mango", 
                                  "orange", "passionfruit", "grape" };

            // Get the only string in the array whose length is greater than 10.
            string fruit1 = fruits.AsQueryable().Single(fruit => fruit.Length > 10);

            Console.WriteLine("First Query: " + fruit1);

            try
            {
                // Try to get the only string in the array
                // whose length is greater than 15.
                string fruit2 = fruits.AsQueryable().Single(fruit => fruit.Length > 15);
                Console.WriteLine("Second Query: " + fruit2);
            }
            catch (System.InvalidOperationException)
            {
                Console.Write("Second Query: The collection does not contain ");
                Console.WriteLine("exactly one element whose length is greater than 15.");
            }

            /*
                This code produces the following output:

                First Query: passionfruit
                Second Query: The collection does not contain exactly one 
                element whose length is greater than 15.
             */



.NET Framework

Supported in: 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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