Sorting DataGrid programmatically

Introduction

This article shows, how to programmatically sort a System.Windows.Forms.DataGrid. In other words, how to emulate a "click" on a column header.

Typically, to sort a DataGrid, we call the DataView.Sort method of an underlying DataView. But what if our DataGrid is bound to a custom datasource? In this case, there's no DataView under our DataGrid!

Background

I was writing an application and the task was to "remember" the way DataGrids are sorted. So I needed to save my sortings to the Windows registry and restore it each time my application starts.

The problem was: my DataGrid was bound to a custom collection, no DataViews or DataTables.

Sorting Basics

If your DataGrid is bound to some IList collection and you want your DataGrid to support sorting - your collection must implement IBindingList interface. This interface contains a method ApplySort() among others. When a user clicks a column header, DataGrid calls the datasource's method ApplySort().

ApplySort() method takes two arguments: property and direction.

void IBindingList.ApplySort(PropertyDescriptor property, 
                                           ListSortDirection direction)

Each column of your DataGrid represents some property of an underlying object. And you have to pass this property to ApplySort() method.

Emulate column header click

I've discovered that simply calling ApplySort() doesn't work. Calling DataGrid's OnMouseDown() protected method with MouseEventArgs pointed to a column header also doesn't work!

So, I used System.Reflection to look deep inside the DataGrid class and see what happens when a user clicks a header. In the list of DataGrid's private members, I've found a private method ColumnHeaderClicked(). See what I mean? Bingo.

This method is defined as follows:

private void ColumnHeaderClicked(PropertyDescriptor prop)

So, if we want to sort column number 5, we have to determine the underlying PropertyDescriptor for this column and pass this PropertyDescriptor to ColumnHeaderClicked method. This can be done in two ways.

• Calling for a GridColumnStyle.PropertyDescriptor property of a grid column. But this property sometimes returns null if our grid is bound to a custom collection through MappingNames.
• If our DataGrid is bound to a custom collection MyCollection, we typically create a TableStyle and some GridColumnStyles and assign its MappingNames. TableStyle's MappingName would be "MyCollection" and ColumnStyle's MappingName would contain the name of some property this column displays. So, we can get the PropertyDescriptor object by seeking for the property with the name, equal to the column's MappingName.

After we have a PropertyDescriptor object, we simply invoke a private method using System.Reflection.

Now the code:

public class MyDataGrid : DataGrid
{
 //sort a column by its index
 public void SortColumn(int columnIndex)
 {
  if(this.DataSource!=null && 
    ((System.Collections.IList)this.DataSource).Count>0)
  {
   //discover the TYPE of underlying objects
   Type sourceType = ((System.Collections.IList)this.DataSource)[0].GetType();

   //get the PropertyDescriptor for a sorted column
   //assume TableStyles[0] is used for our datagrid... (change it if necessary)
   System.ComponentModel.PropertyDescriptor pd =
    this.TableStyles[0].GridColumnStyles[columnIndex].PropertyDescriptor;

   //if the above line of code didn't work try to get a propertydescriptor
   // via MappingName
   if(pd == null)
   {
    System.ComponentModel.PropertyDescriptorCollection pdc =
     System.ComponentModel.TypeDescriptor.GetProperties(sourceType);
    pd =
     pdc.Find( this.TableStyles[0].GridColumnStyles[columnIndex].MappingName, 
                                                                      false);
   }

   //now invoke ColumnHeaderClicked method using system.reflection tools
   System.Reflection.MethodInfo mi =
    typeof(System.Windows.Forms.DataGrid).GetMethod("ColumnHeaderClicked",
     System.Reflection.BindingFlags.Instance |
     System.Reflection.BindingFlags.NonPublic);
   mi.Invoke(this, new object[] { pd });
  }
 }
}

That's it.

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