Error building a content type in SP 2007

Question

I have an event handler that catches an ItemAdding event and changes the content type on the fly. This is my code to build the content type:

SPContentType baseContentType = web.AvailableContentTypes["Folder"];
SPContentType type = new SPContentType(baseContentType, web.ContentTypes, "Custom Folder");

// Add fields to new content type
AddField(ref type, "User", "Owner");
AddField(ref type, "DateTime", "Date");
AddField(ref type, "Text", "Description");
AddField(ref type, "Text", "Justification");
AddField(ref type, "User", "Users");

list.ContentTypes.Add(type);
list.Update();

.. snip ..

void AddField(ref SPContentType contentType, string fieldType, string fieldName)
    {
        SPField field = new SPField(contentType.Fields, fieldType, fieldName);
        if (fieldName.Equals("Users"))
            ((SPFieldUser)field).AllowMultipleValues = true;
        SPFieldLink fieldLink = new SPFieldLink(field);
        contentType.FieldLinks.Add(fieldLink);  // Exception thrown here
        contentType.Update();
    }

On the line: contentType.FieldLinks.Add(fieldLink);,

I'm getting an ArgumentNullException: value cannot be null. Parameter name: g

Does anyone know what this means or can see what I'm doing wrong? Thanks in advance!

Answer 1

it means your SPField field = new ... returned null, as that's what you're trying to pass it into the new SPFieldLink.

If you're trying to create new fields, I strongly recommend using SPFieldCollection.Add() (so, in your example, contentType.Fields.Add()) this will return the internal name of the field just added, so you need to retrieve it again field contentType.Fields and then add it to the FieldLink Collection.

By the way, can you make sure you're not executing this code every time an item is added (you said you're building an event receiver) - as this will start to throw all sorts of errors as soon as you upload your second item.