Validation using IDataError

Question

I have the following validation method in my viewmodel (example is showing only one column, "ItemNumber"):

public bool IsValid
{
  get
  {
    foreach (string property in ValidatedProperties)
      if (GetValidationError(property) != null)
        return false;

    return true;
  }
}

static readonly string[] ValidatedProperties = 
{ 
  "ItemNumber"
};

string GetValidationError(string propertyName)
{
  if (Array.IndexOf(ValidatedProperties, propertyName) < 0)
    return null;

  string error = null;

  switch (propertyName)
  {
    case "ItemNumber":
      error = this.ValidateItemNumber();
      break;
    default:
      Debug.Fail("Unexpected property being validated on ProjectExpense: " + propertyName);
      break;
  }
  // set the status message on the UI to the generated error
  if (error != null)
  {
    ErrorMessage = error;
  }

  return error;
}

// string method
static bool IsStringMissing(string value)
{
  return String.IsNullOrEmpty(value) || value.Trim() == String.Empty;
}

string ValidateItemNumber()
{
  if (SelectedProjectExpenseItem != null)
  {
    if (IsStringMissing(SelectedProjectExpenseItem.ItemNumber))
    {
      return "Item number is required";
    }
    if (SelectedProjectExpenseItem.ItemNumber.Length > 50)
    {
      return "Item number exceeds 50 characters";
    }
  }
  return null;
}

#endregion


#region IDataErrorInfo Members

string IDataErrorInfo.Error { get { return null; } }

string IDataErrorInfo.this[string propertyName]
{
  get { return this.GetValidationError(propertyName); }
}

The validation fires, but I don't know how to communicate it to my datagrid. I am using a separate Dto (hence SelectedProjectExpenseItem.ItemNumber above, SelectedProjectExpenseItem is my Dto) instead of having the properties directly in my viewmodel. Here is my datagrid:

  <DataGrid ItemsSource="{Binding Path=ListOfProjectExpenseItems}" AutoGenerateColumns="False" 
      Name="dgProjectExpenseItems" SelectionMode="Single" SelectionUnit="FullRow" CanUserResizeColumns="True" 
      SelectedItem="{Binding Path=SelectedProjectExpenseItem, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" GridLinesVisibility="Horizontal" CanUserDeleteRows="True" CanUserAddRows="True">
    <DataGrid.Columns>
      <DataGridTextColumn Header="ID" Width="SizeToCells" MinWidth="50" Binding="{Binding RowID}" />
      <DataGridTextColumn Header="Project Expense ID" Width="SizeToCells" Visibility="Hidden" MinWidth="0" Binding="{Binding ProjectExpenseID, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
      <DataGridTextColumn Header="Item Number" Width="SizeToCells" MinWidth="140" Binding="{Binding ItemNumber, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
      <DataGridTextColumn Header="Item Description" Width="SizeToCells" MinWidth="250" Binding="{Binding ItemDescription, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
      <DataGridTextColumn Header="Unit Price" Width="SizeToCells" MinWidth="90" Binding="{Binding ItemUnitPrice, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
      <DataGridTextColumn Header="Qty" Width="SizeToCells" MinWidth="65" Binding="{Binding ItemQty, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
      <DataGridTextColumn Header="Supplier Name" Width="SizeToCells" MinWidth="200" Binding="{Binding SupplierName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
    </DataGrid.Columns>
  </DataGrid>

I would like to use a style, say with a red border and tooltip stating the error, to signify a problem with the row or cell. Every example I found using styles, however, has the properties directly in the viewmodel, and I cannot figure out how to port it over to my Dto.

Any help is appreciated.

Answer 1

The problems had to do with using Dto's in my implementation, or at least the lack of implementing IDataErrorInfo in them. I decided to dump them and go with straight business objects that implement IDataErroInfo and wa-la, everything works beautifully now.