Your custom field type can be added to a list in a browser easily. But how do you add a custom field type via code?

Here is my way:

1. add a new field with the field type from which your custom field type derived
2. change the field type of the new field to your own custom field type

In my case my custom field type derived from a SPFieldLookup.

  1:   // create new lookup field 

   2:  string newFieldName = fields.AddLookup("fieldname", list.ID, web.ID, false); 

   3:  var newField = fields.GetFieldByInternalName(newFieldName); 

   4:  // change field type to our own 

   5:  newField.SchemaXml = newField.SchemaXml.Replace("Lookup", "yourFieldType");

Update 4. Feb 2008

The above solution will bring you a field with the type. But a much smoother way is to create a new field, which has the selected type. This way you don’t need to modify the schema of the field.

1:  CustomFieldClass field = list.Fields.CreateNewField("CustomFieldClass", "The name of the field");

2:  list.Fields.Add(field);