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 : source:IQueryable<'TSource> * predicate:Expression<Func<'TSource, bool>> -> 'TSource
The type of the elements of source.
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.
Usage NoteIn Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable(). 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).
source or predicate is a null reference (Nothing in Visual Basic).
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 method generates a MethodCallExpression that represents calling 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 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.