Sharepoint: Creating custom field types in SharePoint 2007

Good references for creating custom field types

1. http://msdn.microsoft.com/en-us/magazine/dd727508.aspx
2. http://vspug.com/nicksevens/2007/08/31/create-custom-field-types-for-sharepoint/

Troubleshooting :

Issue : After deploying your custom field type , if you try creating a column in a list , you see below error :

The given key was not present in the dictionary. at System.ThrowHelper.ThrowKeyNotFoundException()

at System.Collections.Generic.Dictionary`2.get_Item(TKey key)

at Microsoft.SharePoint.SPFieldCollection.CreateSPField(Int32 index)

at Microsoft.SharePoint.SPFieldCollection.EnsureSPField(Int32 index)

at Microsoft.SharePoint.SPFieldCollection.get_Item(Int32 iIndex)

at Microsoft.SharePoint.ApplicationPages.FieldCustomizationPage.OnPreRender(EventArgs e)

at Microsoft.SharePoint.ApplicationPages.FieldNewPage.OnPreRender(EventArgs e)

at System.Web.UI.Control.PreRenderRecursiveInternal()

at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Resolution : Follow the below steps :

Go the settings page of the list in which you are trying to create the column. If you see any entry as shown in the below screen , go ahead and delete the Garbage field.

If you have deployed any custom field type earlier and removed its references later , these garbage fields get attached with that list.

Issue :

Failed to set value of the "add" column on the "<Your column>" field type control. See details in log. Exception message: Unable to cast object of type 'System.String' to type 'Microsoft.SharePoint.SPFieldMultiColumnValue'..

Resolution :

If you try to edit the list items created using custom field types , you might see the above error.

To fix this error , set the control's values in the CreateChildControls method as below :

if (ControlMode == Microsoft.SharePoint.WebControls.SPControlMode.Edit)

{

txtUserInput = (TextBox)TemplateContainer.FindControl("txtUserInput");

}