Code Sample : How to add a site column to a list

public static string AddFieldToList(SPWeb web, string listName, 

 string fieldName, SPFieldType fieldType,

 bool fieldRequired)

{

    LogHelper.LogDebug(logger, "Begin AddFieldToList");

 string internalListName = string.Empty;

    ValidationHelper.VerifyStringArgument(listName, "listName");

    SPList list = web.Lists[listName];

 if (list != null)

    {

        internalListName = AddFieldToList(web, list, fieldName, fieldType, fieldRequired);

    }

    LogHelper.LogDebug(logger, "End AddFieldToList");

 return internalListName;

}

public static string AddFieldToList(SPWeb web, SPList list, string fieldName, 

                                    SPFieldType fieldType, bool fieldRequired)

{

    LogHelper.LogDebug(logger, "Begin AddFieldToList");

    ValidationHelper.VerifyObjectArgument(web, "web");

    ValidationHelper.VerifyObjectArgument(list, "list");

    ValidationHelper.VerifyStringArgument(fieldName, "fieldName");

 string internalFieldName;

 try

    {

        SPFieldCollection lstFields = list.Fields;

 if (lstFields.ContainsField(fieldName))

        {

 throw new SafException("Field Name:" + fieldName +

 " already exist in the list:" +

                                    list.Title);

        }

        internalFieldName = lstFields.Add(fieldName, fieldType, fieldRequired);

    }

 catch (Exception ex)

    {

        LogHelper.LogError(logger, "SPListHelper", "AddFieldToList", ex);

 throw;

    }

 finally

    {

        LogHelper.LogDebug(logger, "End AddFieldToList");

    }

 return internalFieldName;

}