Iterator pattern in VB.NET (C# would use yield!) [duplicate]

Question

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• Yield in VB.NET 8 answers

How do implement the iterator pattern in VB.NET, which does not have the yield keyword?

Answer 1

This is now supported in VS 2010 SP1, with the Async CTP, see: Iterators (C# and Visual Basic) on MSDN and download Visual Studio Async CTP (Version 3).

Code such as this, works:

Private Iterator Function SomeNumbers() As IEnumerable
    ' Use multiple yield statements.
    Yield 3
    Yield 5
    Yield 8
End Function

Answer 2

VB.NET does not support the creation of custom iterators and thus has no equivalent to the C# yield keyword. However, you might want to look at the KB article How to make a Visual Basic .NET or Visual Basic 2005 class usable in a For Each statement for more information.

Answer 3

C#'s yield keyword forces the compiler to create a state machine in the background to support it. VB.Net does not have the yield keyword. But it does have a construct that would allow you to create a state machine within a function: Static function members.

It should be possible to mimic the effects of a yield return function by creating a generic class that implements IEnumerable as well as the needed state machine and placing an instance as a static member inside your function.

This would, of course, require implementing the class outside of the function. But if done properly the class should be re-usable in the general case. I haven't played with the idea enough to provide any implementation details, though.

Answer 4

Hmm, looks like you might be out of luck:

I was struggling with an issue today when converting some C# to VB.NET. C# has a really cool "yield return" statement that is used in an iterator block to provide a value to the enumerator object. VB.NET does not have the "yield" keyword. So, there are a few solutions (none of which are really clean) to get around this. You could use a return statement to return the value if you are looping through and would like to break an enumerator and return a single value. However, if you'd like to return the entire enumeration, create a List() of the child type and return the list. Since you are usually using this with an IEnumerable, the List() will work nice.

That was written a year ago, not sure if anyone has come up with anything else better since then..

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Edit: this will be possible in the version 11 of VB.NET (the one after VS2010), support for iterators is planned. The spec is available here.

Answer 5

Keep in mind that deferred execution and lazy evaluation properties of LINQ expresssions and methods allow us to effectively implement custom iterators until the yield statement is available in .NET 4.5. Yield is used internally by LINQ expressions and methods.

The following code demonstrates this.

    Private Sub AddOrRemoveUsersFromRoles(procName As String,
                                      applicationId As Integer,
                                      userNames As String(),
                                      rolenames As String())
    Dim sqldb As SqlDatabase = CType(db, SqlDatabase)
    Dim command As DbCommand = sqldb.GetStoredProcCommand(procName)
    Dim record As New SqlDataRecord({New SqlMetaData("value", SqlDbType.VarChar,200)})
    Dim setRecord As Func(Of String, SqlDataRecord) =
        Function(value As String)
            record.SetString(0, value)
            Return record
        End Function
    Dim userNameRecords As IEnumerable(Of SqlDataRecord) = userNames.Select(setRecord)
    Dim roleNameRecords As IEnumerable(Of SqlDataRecord) = rolenames.Select(setRecord)
    With sqldb
        .AddInParameter(command, "userNames", SqlDbType.Structured, userNameRecords)
        .AddInParameter(command, "roleNames", SqlDbType.Structured, roleNameRecords)
        .AddInParameter(command, "applicationId", DbType.Int32, applicationId)
        .AddInParameter(command, "currentUserName", DbType.String, GetUpdatingUserName)
        .ExecuteNonQuery(command)
    End With
End Sub

Answer 6

Below gives output: 2, 4, 8, 16, 32

In VB.NET

Public Shared Function setofNumbers() As Integer()

    Dim counter As Integer = 0
    Dim results As New List(Of Integer)
    Dim result As Integer = 1
    While counter < 5
        result = result * 2
        results.Add(result)
        counter += 1
    End While
    Return results.ToArray()
End Function

Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    For Each i As Integer In setofNumbers()
        MessageBox.Show(i)
    Next
End Sub

In C#

private void Form1_Load(object sender, EventArgs e)
{
    foreach (int i in setofNumbers())
    {
        MessageBox.Show(i.ToString());
    }
}

public static IEnumerable<int> setofNumbers()
{
    int counter=0;
    //List<int> results = new List<int>();
    int result=1;
    while (counter < 5)
    {
      result = result * 2;
      counter += 1;
      yield return result;
    }
}