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

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

Usage Note

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.

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.
            */

.NET Framework

.NET Framework Client Profile

Portable Class Library

.NET for Windows Store apps

Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

Reference