public Form1() {
    InitializeComponent();
    vGridControl1.DataSource = dataSource;
    vGridControl1..ForceInitialize();
    vGridControl1..ShowCustomization();
}

Public Sub New()
   InitializeComponent()
   vGridControl1.DataSource = dataSource;
   vGridControl1.ForceInitialize()
   vGridControl1.ShowCustomization()
End Sub

public Form1() {
    InitializeComponent();
    vGridControl1.DataSource = dataSource;
    vGridControl1..ForceInitialize();
    vGridControl1..ShowCustomization();
}

Public Sub New()
   InitializeComponent()
   vGridControl1.DataSource = dataSource;
   vGridControl1.ForceInitialize()
   vGridControl1.ShowCustomization()
End Sub

using DevExpress.XtraVerticalGrid.Events;

public class RowOperationCategoryCaption : RowOperation {
    private string newCategoryCaption;
    private bool found = false;
    public RowOperationCategoryCaption(string newCaption) {
        this.newCategoryCaption = newCaption;
    }
    public bool Found {
        get { return found; }
    }
    public override void Execute(BaseRow row) {
        // Check if the processed row is a category.
        if (row.XtraRowTypeID != 0) return;
        // Compare the row caption with the entered caption.
        if (row.Properties.Caption == newCategoryCaption) found = true;
    }
   public override bool CanContinueIteration(BaseRow row){
      return !Found;
   }
}

private void vGridControl1_CustomizationFormCreatingCategory(object sender, 
CustomizationFormCreatingCategoryEventArgs e) {
    RowOperationCategoryCaption operation = new RowOperationCategoryCaption(e.Category.Properties.Caption);
    VGridControl vGridControl = sender as VGridControl;
    // Perform the operation.
    vGridControl.RowsIterator.DoOperation(operation);
    // Display a warning message if the match is found.
    if (operation.Found) {
        string messageText = 
            "A category row with the same caption already exists.\r\n" + 
            "Click Yes to modify the new category row caption.\r\n" +
            "Click No to create a new category row with the specified caption.\r\n"+
            "Do you want to enter new caption?";
        DialogResult result = MessageBox.Show(messageText,"Match found",
          MessageBoxButtons.YesNo);
        // Prohibit to create a new row depending on the user choice.
        if (result == DialogResult.Yes) e.CanCreate = false;
    }
}

Imports DevExpress.XtraVerticalGrid.Events

Public Class RowOperationCategoryCaption
    Inherits RowOperation
    Private _newCategoryCaption As String
    Private _found As Boolean = False
    Public Sub New(ByVal NewCaption As String)
        Me._newCategoryCaption = NewCaption
    End Sub

    Public ReadOnly Property Found() As Boolean
        Get
            Return _found
        End Get
    End Property

    Public Overrides Sub Execute(ByVal Row As BaseRow)
        ' Check if the processed row is a category.
        If Row.XtraRowTypeID <> 0 Then Return
        ' Compare the row caption with the entered caption.
        If Row.Properties.Caption = _newCategoryCaption Then _found = True
    End Sub

   Public Overrides Function CanContinueIteration(ByVal row As BaseRow) As Boolean
      Return Not Found
   End Function
End Class

Private Sub VGridControl1_CustomizationFormCreatingCategory(ByVal sender As Object, _
ByVal e As CustomizationFormCreatingCategoryEventArgs) _
Handles VGridControl1.CustomizationFormCreatingCategory
    Dim Operation As New RowOperationCategoryCaption(e.Category.Properties.Caption)
    Dim vGridControl As VGridControl = TryCast(sender, VGridControl)
    ' Perform the operation.
    VGridControl.RowsIterator.DoOperation(Operation)
    ' Display a warning message if the match is found.
    If Operation.Found Then
    Dim MessageText As String = 
      "A category row with the same caption already exists." & Chr(13) & _
      "Click Yes to modify the new category row caption." & Chr(13) & _
      "Click No to create a new category row with the specified caption." & Chr(13) & _
      "Do you want to enter new caption?"
    Dim Result As DialogResult = MessageBox.Show(MessageText, "Match Found", _
      MessageBoxButtons.YesNo)
        ' Prohibit to create a new row depending on the user choice.
        If Result = DialogResult.Yes Then e.CanCreate = False
    End If
End Sub