We can use the Single extension method of LINQ to return the only element in a sequence that satisfies a specified condition. However the Single extension method would throw the System.InvalidOperationException when no element in a sequence satisfies a specified condition. In such a scenario, we can use the SingleOrDefault extenion method which would return the default value of a type instead of throwing the System.InvalidOperationException.

Also, both Single as well as SingleOrDefault extension methods would throw the System.InvalidOperationException when more than one element in a sequence satisfy a specified condition. We can use First or Last extensions methods of LINQ in such scenarios.

Code example (both in C# and VB.NET) with comments below:

C#:

    // Create a new generic list of ints
    List<int> l = new List<int>();

    l.Add(1); // Add 1 to the list
    l.Add(5); // Add 5 to the list
    l.Add(3); // Add 3 to the list

    // Returns 1 as only 1 satisfies the condition
    int value = l.Single(i => i == 1);

    // Returns the default value of int which is 0
    // since no element in the list equals 4
    value = l.SingleOrDefault(i => i == 4);

    // Throws System.InvalidOperationException
    // since no element in the list equals 4
    value = l.Single(i => i == 4);

    // Throws System.InvalidOperationException
    // since both 5 and 3 are greater than 1
    value = l.Single(i => i > 1);
    value = l.SingleOrDefault(i => i > 1);

VB.NET: 

    ' Create a new generic list of ints
    Dim l As New List(Of Integer)

    l.Add(1) ' Add 1 to the list
    l.Add(5) ' Add 5 to the list
    l.Add(3) ' Add 3 to the list

    ' Returns 1 as only 1 satisfies the condition
    Dim value As Integer = l.Single(Function(i) i = 1)

    ' Returns the default value of int which is 0
    ' since no element in the list equals 4
    value = l.SingleOrDefault(Function(i) i = 4)

    ' Throws System.InvalidOperationException
    ' since no element in the list equals 4 
    value = l.Single(Function(i) i = 4)

    ' Throws System.InvalidOperationException
    ' since both 5 and 3 are greater than 1
    value = l.Single(Function(i) i > 1)
    value = l.SingleOrDefault(Function(i) i > 1)

Cheers,
Raj

~~~ CODING FOR ETERNITY !!! ~~~