Complex Data Binding a Collection Implementing IBindingList and ITypedList

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Introduction

Data binding can be nasty - especially when you can't use the System.Data objects. I have tried to use all the features of data binding to show them to you. It is pretty small, but takes a hell of a long way to get there.

The sample shows a custom collection with objects in it having properties, some of those properties are collections again, with objects in them - i.e., hierarchical data. Maybe some of the properties can't be edited directly because the grid is not smart enough - like SqlTypes.

The Problem

We have a collection of objects that need to go into a Windows DataGrid. They may or may not have properties that are collections too.

The Solution

We need to create the following objects:

• A class implementing ITypedList and IBindingList (your collection)
• A class that is your data (lots of properties and stuff)
• A class implementing IComparer (for sorting)
• A class implementing PropertyDescriptor (for using non grid compatible types, like SqlTypes)
• A class implementing Attribute (so we know what's in the collection)

For those of you that have tried this and it didn't work, a couple of issues came up that I will discuss.

ITypedList.GetItemProperties(PropertyDescriptor[] listAccessors) 

This was the nastiest method interface in the project. The problem had to do with the listAccessors. If the parameter is not null, then the only one that should be processed is the last one. I had to decompile System.Data.DataView.ITypedList.GetItemProperties to find that out. Then I needed to detect our custom collections and return the properties for the underlying data.

SqlPropertyDescriptor : PropertyDescriptor

This one, although straight forward to implement, is a bit tricky in that when calling GetItemProperties and we want to use this PropertyDesciptor instead of the default, we need to replace it in the collection. I wrote this method to do it:

protected PropertyDescriptorCollection 
    GetPropertyDescriptorCollection( ArrayList properties )
{
    if ( properties == null || properties.Count == 0 )
        return new PropertyDescriptorCollection(null);

    ArrayList output = new ArrayList();

    foreach ( PropertyDescriptor p in properties )
    {
        if ( p.Attributes.Matches(new Attribute[]
            {new BindableAttribute(false)}) ) continue;

        if ( p.PropertyType.Namespace == "System.Data.SqlTypes" )
        {
            // create the base type property descriptor
            output.Add(SqlPropertyDescriptor.
                GetProperty( p.Name, p.PropertyType ) );
        }
        else
        {
            output.Add(p);
        }
    }
    return new PropertyDescriptorCollection((PropertyDescriptor[])
        output.ToArray(typeof(PropertyDescriptor)));
}

After that, it was all straight forward.

The SetValue and GetValue of the PropertyDescriptor provide the means to edit SqlTypes like SqlDateTime. After detecting the namespace of the data type, I use SqlPropertDescriptor which implements GetValue() and SetValue(). These methods try to figure everything out for themselves:

public override void SetValue(object component,object value)
{
    try
    {
        PropertyInfo pi = component.GetType().GetProperty(this.Name);
        Object o;
        if ( value == DBNull.Value )
        {
            o = component.GetType().GetField("Null", 
                BindingFlags.Static | BindingFlags.Public | 
                BindingFlags.GetField).GetValue(component);
                
        }
        else
        {
            o = pi.PropertyType.GetConstructor(new Type[]
                {BaseType}).Invoke(new Object[]{value});
        }
        pi.SetValue(component,o, null);
    }
    catch(Exception ex)
    {
        Debug.WriteLine(ex);
    }
}

public override object GetValue(object component)
{
    try
    {
        object Property = component.GetType().GetProperty
            (this.Name).GetValue(component,null);

        // handle nulls
        if ( (bool)Property.GetType().GetProperty
            ("IsNull").GetValue(Property,null) ) 
            return DBNull.Value;

        object Value = Property.GetType().GetProperty
            ("Value").GetValue(Property,null);
        return Value;
    }
    catch(Exception ex)
    {
        Debug.WriteLine(ex);
    }
    return null;
}

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