显示SharePoint自定义字段类型的自定义属性值时出现问题

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这是我的fldtypes*.xml文件:

<?xml version="1.0" encoding="utf-8"?>
<FieldTypes>
  <FieldType>
    <Field Name="TypeName">MyCustomField</Field>
    <Field Name="TypeDisplayName">My Custom Field</Field>
    <Field Name="TypeShortDescription">MyCustomField</Field>
    <Field Name="ParentType">Text</Field>
    <Field Name="UserCreatable">TRUE</Field>
    <Field Name="FieldTypeClass">MyCustomField.CustomFields.MyCustomField, MyCustomField, Version=1.0.0.0, Culture=neutral, PublicKeyToken=d772gbab82fe6996</Field>
    <PropertySchema>
      <Fields>
        <Field Name="MyCustomProp" DisplayName="My Custom Prop" Type="Text" MaxLength="50" DisplaySize="30" />
      </Fields>
    </PropertySchema>
  </FieldType>
</FieldTypes>

这是我的字段类型类的代码:

public class MyCustomField : SPFieldText
{
    private string propval;

    public MyCustomField(SPFieldCollection fields, string fieldName)
        : base(fields, fieldName)
    {
    }

    public MyCustomField(SPFieldCollection fields, string typeName, string displayName)
        : base(fields, typeName, displayName)
    {           
    }

    public override void Update()
    {
        // I can see any updated value here
        propval = GetCustomProperty("MyCustomProp") as string;
        base.Update();
    }

    public override Type FieldValueType
    {
        get { return typeof (string); }
    }

}

如何在SharePoint应用程序的“更改列”页面(FdEditEx.aspx)中查看正确的自定义属性值?