How can I import a third party .net dll into my CLR Function?

I need to import a dll into my clr function. Because early import is not supported I try to late import my DLL:
If I try to deploy I get an error without error Nr. The Message is (translatet from German):
Fehler beim Erstellen.
Error while build - but not more.
using System;
using System.Data;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;
using System.Runtime.InteropServices;

public partial class UserDefinedFunctions
    public static extern string DoSomething(string something);

    public static SqlString Go(string myString)
        string plain = DoSomething(myString);
        return new SqlString(plain);

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don't know if this is really what you are looking for, but thought I'd give it a try:
OpusretisAuthor Commented:

No these are basics in "normal" projects. I have a CLR stored User defined Function for SQL Server.

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