Overload DataGridViewCellStyle and give a default value

Question

I'm writing a custom DataGridView object for a large project to hand out to a bunch of developers to make our app sections look consistent.

I want to set defaults for many of the properties of the DataGridView, and I can set many of them like this:

<System.ComponentModel.Browsable(True), System.ComponentModel.DefaultValue(DataGridViewAutoSizeColumnsMode.Fill)>_
Public Overloads Property AutoSizeColumnsMode() As DataGridViewAutoSizeColumnMode
    Get
        Return MyBase.AutoSizeColumnsMode
    End Get
    Set(ByVal value As DataGridViewAutoSizeColumnMode)
        MyBase.AutoSizeColumnsMode = value
    End Set
End Property

These properties overload with their defaults just fine. Its when I started trying to make default Cell styles that I ran into the issue. Since the DataGridViewCellStyle is a class, I cannot make a constant out of it. I've tried changing all of the settings to what I want them to be in the class constructor, and that works great, except that changes made in the designer properties just get set back as soon as the app runs. So putting the changes in the constructor won't do.

Is there anywhere else I can put code that only runs when the control is first dropped on the designer? or any other way of setting a default?

Answer 1

Actually, I thought about it a while longer and came across a simpler solution for my issue. This does not work for all cases because it relies on the fact that the person using the custom component will likely never want to revert an entire CellStyle back to windows defaults. I ended up comparing a new CellStyle to the current one in the constructor, and only setting the style if they matched. This way it won't overwrite changes, but it will set it up the first time.

Public Class CustomDataGridView
    Inherits System.Windows.Forms.DataGridView
    Private RowStyle As New DataGridViewCellStyle

    Public Sub New()
        RowStyle.BackColor = Color.FromArgb(223, 220, 200)
        RowStyle.Font = New Font("Arial", 12.75, FontStyle.Bold, GraphicsUnit.Point)
        RowStyle.ForeColor = Color.Black
        RowStyle.SelectionBackColor = Color.FromArgb(94, 136, 161)

        If MyBase.RowsDefaultCellStyle.ToString = (New DataGridViewCellStyle).ToString Then
            MyBase.RowsDefaultCellStyle = RowStyle
        End If 
    End Sub
End Class

Just goes to show, Just because you have a golden hammer, doesn't mean that every problem is a nail.

Answer 2

I ran into this problem too. My solution works around the requirement for the DefaultValue argument to be a compile-time constant. I thought, wouldn't it be sufficient to set the value in the class constructor (defined by the static constructor in C#, and the shared constructor in VB) instead?

This seems to be an good work-around in my case, though there are probably instances where it might break since it's not actually present in the meta-data until the class constructor is called upon loading the class, but for a Designer attribute that should be acceptable. Because DefaultValueAttribute.SetValue is protected, I had to define a derived class that makes it public.

This works fine in the designer, it recognizes when the value is the same as the default and omits it from the generated code when possible, and only generates the differences from the default.

Here's the code in C#, this should work in VB too but I'm not overly familiar with its syntax so I'll have to leave that up to you.

public partial class HighlightGrid : DataGridView
{
    // Class constructor
    static MethodGrid()
    {
        // Get HighlightStyle attribute handle
        DefaultValueSettableAttribute attr =
            TypeDescriptor.GetProperties(typeof(HighlightGrid))["HighlightStyle"]
            .Attributes[typeof(DefaultValueSettableAttribute)]
            as DefaultValueSettableAttribute;

        // Set default highlight style
        DataGridViewCellStyle style = new DataGridViewCellStyle();
        style.BackColor = Color.Chartreuse;
        attr.SetValue(style);
    }
    [DefaultValueSettable, Description("Cell style of highlighted cells")]
    public DataGridViewCellStyle HighlightStyle
    {
        get { return this.highlightStyle; }
        set { this.highlightStyle = value; }
    }
    // ...
}

// Normally the value of DefaultValueAttribute can't be changed and has
// to be a compile-time constant. This derived class allows setting the
// value in the class constructor for example.
public class DefaultValueSettableAttribute : DefaultValueAttribute
{
    public DefaultValueSettableAttribute() : base(new object()) { }
    public new void SetValue(Object value) { base.SetValue(value); }
}