IsMissing Support

The IsMissing function does not exist in .NET. However VB6 AI Migrator can support the conversion of IsMissing by implementing certain conversion patterns.

The VB6 AI Migrator starts by recognizing the IsMissing cases that will never return true. These cases are converted to the literal boolean “false”. Other cases are converted by using a specific conversion that provides the same behavior in .NET. It includes optional parameters to overloadings, default values deduction, specific parameter types to general types (nullable for value-types and object for references) and auxiliary local variables generation.

The following basic example illustrates this specific conversion:

Original VB6 Code:

Sub TestIsMissing(Optional i1, Optional f)
   If IsMissing(i1) Then
      MsgBox "i1 is Missing"
   End If
   If IsMissing(f) Then
      MsgBox "f is Missing"
   End If
End Sub

Sub Main()
   Dim f As Form
   TestIsMissing 1000, f
   TestIsMissing 1000
   TestIsMissing
End Sub

Resulting VB.NET Code:

Sub TestIsMissing(ByRef i1_optional As Nullable(Of Integer), ByRef f_optional As Object)
    Dim i1 As Integer = 0
    If i1_optional.HasValue Then i1 = i1_optional.Value
    Dim f As Form = Nothing
    If f_optional Is Nothing OrElse Not f_optional.Equals(Type.Missing) Then f = TryCast(f_optional, Form)
    If Not i1_optional.HasValue Then
        MessageBox.Show("i1 is Missing", Application.ProductName)
    End If
    If Not (f_optional Is Nothing) AndAlso f_optional.Equals(Type.Missing) Then
        MessageBox.Show("f is Missing", Application.ProductName)
    End If
End Sub

Sub TestIsMissing(ByRef i1_optional As Nullable(Of Integer))
    Dim tempRefParam As Object = Type.Missing
    TestIsMissing(i1_optional, tempRefParam)
End Sub

Sub TestIsMissing()
    Dim tempRefParam2 As Nullable(Of Integer) = Nothing
    Dim tempRefParam3 As Object = Type.Missing
    TestIsMissing(tempRefParam2, tempRefParam3)
End Sub

Public Sub Main()
    Dim f As Form
    TestIsMissing(1000, f)
    TestIsMissing(1000)
    TestIsMissing()
End Sub

Resulting C#.NET Code:

static public void  TestIsMissing( int? i1_optional, object f_optional)
{
    int i1 = ((i1_optional == null) ? 0: i1_optional.Value);
    Form f = ((f_optional == Type.Missing) ? null: f_optional as Form);
    if (i1_optional == null)
    {
        MessageBox.Show("i1 is Missing", Application.ProductName);
    }
    if (f_optional == Type.Missing)
    {
        MessageBox.Show("f is Missing", Application.ProductName);
    }
}

static public void  TestIsMissing( int? i1_optional)
{
    TestIsMissing(i1_optional, Type.Missing);
}

static public void  TestIsMissing()
{
    TestIsMissing(null, Type.Missing);
}

static public void  Main()
{
    Form f = null;
    TestIsMissing(1000, f);
    TestIsMissing(1000);
    TestIsMissing();
}