SingleOrDefault LINQ Sample

SingleOrDefault

SingleOrDefault: Retrieves only element in a collection, or default value if collection is empty.
SingleOrDefault: Retrieves only element in a collection, or default value if collection is empty.

This Lambda Expression sample retrives a single element from array, but from arrays with not exactly one element it gets default value (null).
This Lambda Expression sample retrives a single element from array, but from arrays with not exactly one element it gets default value (null).

// A Query Expression cannot be constructed for SingleOrDefault() in C#.
// Consider using a Lambda Expression instead.

' A Query Expression cannot be constructed for SingleOrDefault() in VB.NET.
' Consider using a Lambda Expression instead.

// There is no SingleOrDefault() for Query Expressions in F#, but it has the similar "exactlyOneOrDefault".
let Sample_SingleOrDefault_Linq() =
    let names1 = [| "Peter" |]
    let names3 = [| "Peter"; "Joe"; "Wilma" |]
    let empty:System.String list = []

    let result1 = query {
        for n in names1 do
        exactlyOneOrDefault
    }

    let resultEmpty = query {
        for n in empty do
        exactlyOneOrDefault
    }
    
    Debug.WriteLine(sprintf "The only name in the array is:")
    Debug.WriteLine(sprintf "%s" result1)

    Debug.WriteLine(sprintf "As array is empty, exactlyOneOrDefault yields null:")
    Debug.WriteLine(sprintf "%b" (resultEmpty = null))

    try
        // This will throw an exception as well because array contains more than one element
        let result3 = query {
            for n in names3 do
            exactlyOneOrDefault
        }
        ()
    with
        | e -> Debug.WriteLine(sprintf "%s" e.Message)

Output:

The only name in the array is:
Peter
As array is empty, exactlyOneOrDefault yields null:
true
A first chance exception of type 'System.InvalidOperationException' occurred in System.Core.dll
Sequence contains more than one element

// Note: SingleOrDefault retrieves null value if array is empty, and...
//       ...throws an exception if array contains more than one element.
static void Sample_SingleOrDefault_Lambda()
{
    string[] names1 = { "Peter" };
    string[] names3 = { "Peter", "Joe", "Wilma" };
    string[] empty = { };

    var result1 = names1.SingleOrDefault();

    var resultEmpty = empty.SingleOrDefault();

    Debug.WriteLine("The only name in the array is:");
    Debug.WriteLine(result1);

    Debug.WriteLine("As array is empty, SingleOrDefault yields null:");
    Debug.WriteLine(resultEmpty == null);

    try
    {
        // This will throw an exception as well because array contains more than one element
        var result3 = names3.SingleOrDefault();
    }
    catch (Exception e)
    {
        Debug.WriteLine(e.Message);
    }
}

Output:

The only name in the array is:
Peter
As array is empty, SingleOrDefault yields null:
True
A first chance exception of type 'System.InvalidOperationException' occurred in System.Core.dll
Sequence contains more than one element

' Note: SingleOrDefault retrieves null value if array is empty, and...
'       ...throws an exception if array contains more than one element.
Private Shared Sub Sample_SingleOrDefault_Lambda()
    Dim names1 As String() = {"Peter"}
    Dim names3 As String() = {"Peter", "Joe", "Wilma"}
    Dim empty As String() = {}

    Dim result1 = names1.SingleOrDefault()

    Dim resultEmpty = empty.SingleOrDefault()

    Debug.WriteLine("The only name in the array is:")
    Debug.WriteLine(result1)

    Debug.WriteLine("As array is empty, SingleOrDefault yields null:")
    Debug.WriteLine(resultEmpty Is Nothing)

    Try
        ' This will throw an exception as well because array contains more than one element
        Dim result3 = names3.SingleOrDefault()
    Catch e As Exception
        Debug.WriteLine(e.Message)
    End Try
End Sub

Output:

The only name in the array is:
Peter
As array is empty, SingleOrDefault yields null:
True
A first chance exception of type 'System.InvalidOperationException' occurred in System.Core.dll
Sequence contains more than one element

let Sample_SingleOrDefault_Lambda() =
    let names1 = [| "Peter" |]
    let names3 = [| "Peter"; "Joe"; "Wilma" |]
    let empty:System.String list = []

    let result1 = names1.SingleOrDefault()

    let resultEmpty = empty.SingleOrDefault()

    Debug.WriteLine(sprintf "The only name in the array is:")
    Debug.WriteLine(sprintf "%s" result1)

    Debug.WriteLine(sprintf "As array is empty, SingleOrDefault yields null:")
    Debug.WriteLine(sprintf "%b" (resultEmpty = null))

    try
        // This will throw an exception as well because array contains more than one element
        let result3 = names3.SingleOrDefault()
        ()
    with
        | e -> Debug.WriteLine(sprintf "%s" e.Message)

Output:

The only name in the array is:
Peter
As array is empty, SingleOrDefault yields null:
true
A first chance exception of type 'System.InvalidOperationException' occurred in System.Core.dll
Sequence contains more than one element

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SingleOrDefault