How to call delphi's dll in C#? [duplicate]

Question

The delphi dll code like this:

function TCP_CHECKOUT(const RoomName:string; ClearCard:boolean):Byte

when I call it in c#,but always get the error return code. My c# code like this:

[DllImport(@"EHLKEY.dll", EntryPoint = "TCP_CHECKOUT")]
public extern static Byte TCP_CHECKOUT(string RoomName, bool ClearCard);

The delphi code user said the return code 255 is not call success.Somebody can help me to solve it?Thanks!