Queryable.FirstOrDefault<'TSource> Method (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.
Assembly: System.Core (in System.Core.dll)
static member FirstOrDefault<'TSource> : source:IQueryable<'TSource> * predicate:Expression<Func<'TSource, bool>> -> 'TSource
Parameters
- source
-
Type:
System.Linq.IQueryable<'TSource>
An IQueryable<'T> to return an element from.
- predicate
-
Type:
System.Linq.Expressions.Expression<Func<'TSource, Boolean>>
A function to test each element for a condition.
Return Value
Type: TSourcedefault(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.
Type Parameters
- TSource
The type of the elements of source.
| Exception | Condition |
|---|---|
| ArgumentNullException | source or predicate is null. |
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.
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1