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Databinding the SqlTypes

, 1 May 2003
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Databinding the SqlTypes using the PropertyDescriptor class and the ITypedList Interface

Introduction

Lets say we have an object, which wraps some data from the database. Eventually we want a collection of them displayed in a DataGrid or some other bindable component. For the purpose of this discussion, we will have a class that wraps a DataRow, and properties that wrap its cells.

public class DataRowWrapper
{
  private DataRow dataRow;

  public DataRowWrapper(DataRow dr)
  {
    this.dataRow = dr;
  }

  public int ID
  {
    get { return (int) this.dataRow["ID"]; }
  }

  public DateTime dtStamp
  {
    get { return (DateTime) this.dataRow["dtStamp"]; }
    set { this.dataRow["dtStamp"] = value; }
  }
}

The Problem

This example may look all well and good, but unfortunately a cell can be null, and an int or DateTime cannot! Herein lies the problem. This code will throw exceptions on any null data, and we cannot assign null to the values. So we can use SqlTypes which allow null values.

public SqlDateTime dtStamp
{
  get
  {
    if (this.dataRow.IsNull("dtStamp"))
      return SqlDateTime.Null;
    else
      return new SqlDateTime((DateTime)this.dataRow["dtStamp"]);
  }
  set
  {
    if ( value.IsNull )
      this.dataRow["dtStamp"] = DBNull.Value;
    else
      this.dataRow["dtStamp"] = value.Value;
  }
}

So now we have ruined it for data binding. Data binding does not work for SqlTypes. SqlTypes are not editable. I see this as a big oversight, but there is a solution - PropertyDescriptors.

The Solution

Property descriptors allow us to expose the SqlTypes underlying type ONLY through data binding, and leave everything else the same for normal use of the object. One problem with property descriptors is that we need to implement ITypedList for it to be any use. ITypedList contains the method signature GetItemProperties(), which we can return our own property descriptors with. We could write one property descriptor for each of the SqlTypes, but here’s an easier way.

public class SqlPropertyDescriptor : PropertyDescriptor
{
  public static SqlPropertyDescriptor GetProperty(string name, Type sqlType)
  {
    Type baseType = sqlType.GetProperty("Value").PropertyType;
    ArrayList attribs = new ArrayList(TypeDescriptor.GetAttributes(baseType));
    Attribute[] attrs = (Attribute[])attribs.ToArray(typeof(Attribute));
    return new SqlPropertyDescriptor(name,attrs,sqlType,baseType);
  }

  private Type SqlType;
  private Type BaseType;

  protected SqlPropertyDescriptor( string name,Attribute[] attrs,
    Type sqlType, Type baseType ) : base(name,attrs)
  {
    SqlType = sqlType;
    BaseType = baseType;
  }

  ...

  public override void SetValue(object component,object value)
  {
    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);
  }

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

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

    return Property.GetType().GetProperty("Value").GetValue(Property,null);
  }
}

Here we can see that I have overridden the SetValue() and GetValue(), and since they pass around the data type object, they can handle nulls. I use some reflection to achieve the desired result. SetValue() can put null into the property, and GetValue() can return a null if need be. To use the property descriptors, we must implement ITypedList in our collection, and return our custom property descriptor for all the properties where they are SqlTypes. In my code, I call the following method to replace the property descriptors.

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)));
}

So, now we have a class with properties that are database and null friendly, we can show it in a grid and edit it, and the methods written are generic enough that we can use then on all the SqlTypes.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

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About the Author

Dan Glass
Architect support.com
Australia Australia

Comments and Discussions

 
GeneralSetting Values to Null Using Binding PinmemberSharZoe5-Jun-03 6:04 

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