Vb.net combo box loop

A Form can hold one or more controls in a Collection[^]. You can see this by looking at the Form.Designer.vb class file. For example:

ComboBox1 = New ComboBox()
Button1 = New Button()
TextBox1 = New TextBox()
ComboBox2 = New ComboBox()

' more controls are created here when you add a control to a form

SuspendLayout()
' 
' ComboBox1
' 
ComboBox1.FormattingEnabled = True
ComboBox1.Location = New Point(300, 49)
ComboBox1.Name = "ComboBox1"
ComboBox1.Size = New Size(104, 23)
ComboBox1.TabIndex = 0
' 
' Button1
' 
Button1.Location = New Point(349, 108)
Button1.Name = "Button1"
Button1.Size = New Size(119, 27)
Button1.TabIndex = 1
Button1.Text = "Button1"
Button1.UseVisualStyleBackColor = True
' 
' TextBox1
' 
TextBox1.Location = New Point(357, 160)
TextBox1.Name = "TextBox1"
TextBox1.Size = New Size(91, 23)
TextBox1.TabIndex = 2
' 
' ComboBox2
' 
ComboBox2.FormattingEnabled = True
ComboBox2.Location = New Point(348, 214)
ComboBox2.Name = "ComboBox2"
ComboBox2.Size = New Size(104, 23)
ComboBox2.TabIndex = 3

' more controls are initialized here

' 
' Form1
' 
AutoScaleDimensions = New SizeF(7.0F, 15.0F)
AutoScaleMode = AutoScaleMode.Font
ClientSize = New Size(800, 450)

' the other controls are added here

Controls.Add(ComboBox2)
Controls.Add(TextBox1)
Controls.Add(Button1)
Controls.Add(ComboBox1)
Name = "Form1"
Text = "Form1"
ResumeLayout(False)
PerformLayout()

So we can see when a control is placed on the form, the VS IDE creates the code automatically. Each control is added to the Form1.Controls collection property.

So we need to iterate over this Collection[^] property to access the controls on the form. We can also filter the collection to only return a specific type by using Linq[^] and the OfType[^] extension method:

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

    For Each comboBox As ComboBox In Me.Controls.OfType(Of ComboBox)
        ' do work here on each combobox
        Debug.WriteLine(comboBox.Name)
    Next

End Sub

The code above will iterate over only the ComboBoxes on the form, and will display their names in the Locals window. Replace my code with yours.

UPDATE

Here is Microsoft's implementation of the OfType method:

public static IEnumerable<TResult> OfType<TResult>(this IEnumerable source)
{
	if (source == null)
	{
		ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
	}

	return OfTypeIterator<TResult>(source);
}

private static IEnumerable<TResult> OfTypeIterator<TResult>(IEnumerable source)
{
	foreach (object? obj in source)
	{
		if (obj is TResult result)
		{
			yield return result;
		}
	}
}

Here is a VB translation of the Microsoft code above:

Public Shared Function OfType(Of TResult)(ByVal source As IEnumerable) _
As IEnumerable(Of TResult)

    If source Is Nothing Then
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source)
    End If

    Return OfTypeIterator(Of TResult)(source)

End Function

Private Shared Iterator Function OfTypeIterator(Of TResult)(ByVal source As IEnumerable) _
As IEnumerable(Of TResult)

    For Each obj As Object In source
        If TypeOf obj Is TResult Then
            Dim result As TResult = DirectCast(obj, TResult)
            Yield result
        End If
    Next

End Function

Here you can see the test is the same as the code that @CHill60 has posted, but as a generic test:

If TypeOf obj Is TResult Then
    ' true result
End If

So, TResult is ComboBox and obj is the control in the form's Controls collection.