How to validate the AddNewRow value based on existing records in WinUI DataGrid (SfDataGrid)?

You can validate the newly adding row value based on the existing records by using the CurrentCellValidating event in WinUI DataGrid (SfDataGrid).

//Event subscription
this.dataGrid.CurrentCellValidating += OnCurrentCellValidating;

//Event customization
private void OnCurrentCellValidating(object sender, CurrentCellValidatingEventArgs args)
{
    // Check whether the row is in add new row or not
    bool isAddNewRow = dataGrid.IsAddNewIndex(dataGrid.SelectionController.CurrentCellManager.CurrentRowColumnIndex.RowIndex);
    if (!isAddNewRow)
        return;

    // Check whether the column is CustomerName or not
    if (args.Column.MappingName == "CustomerName")
    {
        // Get the value of the cell
        var text = args.NewValue.ToString();
        // Check whether the value is already existing or not
        var datacontext = (this.dataGrid.DataContext as ViewModel).Orders;
        var listOfNames = datacontext.Where(e => e.CustomerName.Equals(text)).ToList();
        if (listOfNames.Count > 0)
        {
            args.ErrorMessage = "Entered Name is  already existing";
            args.IsValid = false;
        }
    }
}        

You can also use the RowValidating event to validate the row when the cell is edited. The RowValidating event occurs when the edited cells try to commit the row data or lose the focus.

 //Event subscription
 this.dataGrid.RowValidating += OnRowValidating;

 //Event customization
 private void OnRowValidating(object sender, RowValidatingEventArgs e)
 {
     // Check whether the row is in add new row or not
     bool isAddNewRow = dataGrid.IsAddNewIndex(dataGrid.SelectionController.CurrentCellManager.CurrentRowColumnIndex.RowIndex);
     if (!isAddNewRow)
         return;
         
     // Get the value of the column
     var data = e.RowData.GetType().GetProperty("CustomerName").GetValue(e.RowData);

     // Check whether the value is already existing or not
     var datacontext = (this.dataGrid.DataContext as ViewModel).Orders;
     var listOfNames = datacontext.Where(e => e.CustomerName.Equals(data)).ToList();
     if (listOfNames.Count > 0)
     {                
         e.IsValid = false;
         e.ErrorMessages.Add("CustomerName", "Entered Name is  already existing");
     }
 }