Creating Custom Field Control

In this example we are creating a custom field type which concatenates two text fields (first and last name).
Create a new usercontrol file (customfieldcontrol.ascx). In this usercontrol, add a SharePoint:RenderingTemplate control, and insert whatever you want for your control template.

Secondly, create a new class file (customfieldcontrol.cs), which will take the role of codebehind file of the usercontrol. In this class, which will inherit from Microsoft.SharePoint.WebControls.BaseFieldControl, we will override some of the properties and methods to implement our own logic.
using System;
using System.Collections.Generic;
using System.Text;
using System.Web.UI.WebControls;

using Microsoft.SharePoint;
using Microsoft.SharePoint.WebControls;

namespace CustomControl
{
public class customfieldcontrol : BaseFieldControl
{
protected TextBox txtFirstName;
protected TextBox txtLastName;

protected override string DefaultTemplateName
{
get { return "CustomFieldRendering"; }
}
public override object Value
{
get
{
EnsureChildControls();
return txtFirstName.Text + "%" + txtLastName.Text;
}
set
{
try
{
EnsureChildControls();
txtFirstName.Text = value.ToString().Split('%')[0];
txtLastName.Text = value.ToString().Split('%')[1];
}
catch { }
}
}
public override void Focus()
{
EnsureChildControls();
txtFirstName.Focus();
}
protected override void CreateChildControls()
{
if (Field == null) return;
base.CreateChildControls();

//Don't render the textbox if we are just displaying the field
if (ControlMode == Microsoft.SharePoint.WebControls.SPControlMode.Display) return;

txtFirstName = (TextBox)TemplateContainer.FindControl("txtFirstName");
txtLastName = (TextBox)TemplateContainer.FindControl("txtLastName");
if (txtFirstName == null) throw new NullReferenceException("txtFirstName is null");
if (txtLastName == null) throw new NullReferenceException("txtLastName is null");
if (ControlMode == Microsoft.SharePoint.WebControls.SPControlMode.New)
{
txtFirstName.Text = "";
txtLastName.Text = "";
}
}
}
}Actually, what this code does is:Define the ID of the renderingtemplate control in our usercontrol.Set the return value to the value we want it to return :), in this case this will be something like Firstname%Lastname.Make sure that when you edit an item, the proper values are filled in into the textboxes again.
Next thing to do is to create the Field type class (CustomField.cs) itself. In this class, which derives of one of the base control types from SharePoint (SPFieldText, SPFieldChoice, ...), we will define which control has to be used as template, and which value has to be returned when displaying a list item.
using Microsoft.SharePoint;
using Microsoft.SharePoint.WebControls;

namespace CustomControl
{
public class CustomField : SPFieldText
{
public CustomField(SPFieldCollection fields, string fieldName)
: base(fields, fieldName)
{ }

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

public override BaseFieldControl FieldRenderingControl
{
get
{
BaseFieldControl fieldControl = new customfieldcontrol();
fieldControl.FieldName = this.InternalName;
return fieldControl;
}
}

public override string GetValidatedString(object value)
{
return value.ToString().Split('%')[1].ToUpper() + " " + value.ToString().Split('%')[0];
}
}
}Main thing of this code is the GetValidatedString() function. This function defines what is to be displayed when you display an item (in a list view for example).In our case, which in our case will be something like LASTNAME Firstname.
Last file to create is the XML File (fldtypes_custom.xml), which will add the custom field type to SharePoint.



CustomField
Text
Custom Name Field
Custom Name Text Field
TRUE
TRUE
TRUE
TRUE
TRUE
TRUE
TRUE
CustomControl.CustomField, CustomControl, Version=1.0.0.0, Culture=neutral, PublicKeyToken=xxxxxxxxxxxxxxxx
nvarchar


So far so good with creating the files :) Now, all we have to do is put the right files in the right places ...Compile your project (or at least both .cs classes) and make sure they are strong named.Then, install them in the GAC (%windir%\assembly)Next, copy the .ascx file to C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\CONTROLTEMPLATESAnd the last file to copy: copy the .xml file to C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\XML
That's it. Maybe do an IISRESET, and you can use your newly created Custom Field Type!

As taken from Nick's SharePoint Blog. Another good practical example is:

http://www.kcdholdings.com/blog/?p=56