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
{
    [DllImport(@"c:\temp\my.dll")]
    public static extern string DoSomething(string something);

    [Microsoft.SqlServer.Server.SqlFunction]
    public static SqlString Go(string myString)
    {
        string plain = DoSomething(myString);
        return new SqlString(plain);
    }
};

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