How to Successfully Return Strings from Delphi DLL to C# API

Learn how to return strings of unknown length from a Delphi DLL to a C# API with this comprehensive guide, including code examples and step-by-step solutions.
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Introduction

Interoperability between programming languages can sometimes be a complex process, especially when dealing with string data. In this guide, we'll explore a common issue faced by developers: returning strings from a Delphi DLL to a C# API. Specifically, we'll focus on how to manage strings of unknown length. If you’ve been struggling to get this to work in your projects, this guide is for you!

The Problem

You are working with a Delphi DLL and need to return strings back to a C# application. The challenge arises when dealing with strings of unknown length, which cannot be handled easily with standard return types.

The initial approach you've tried is using out parameters, which worked for fixed-sized buffers. However, for strings whose lengths are not predetermined, you need to find a suitable method to manage memory and handle the data correctly.

Initial Code Examples

To better understand the context, here are snippets from both your Delphi functions and C# import statements:

Delphi Functions

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C# Definitions

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Here you encountered runtime errors such as "Runtime error 203" when testing, indicating that the approach required refinement.

The Solution

In order to successfully return strings of unknown length from Delphi to C# , we can employ an alternative approach: using pointers and manual memory management. Here's how to implement the solution.

Step 1: Modify Delphi Functions

Change the signature of the Delphi function to accept and return pointers instead of string types. This allows flexibility in returning string data of any length.

Updated Delphi Code

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In this code snippet, the TestWide function can now handle any length of string passed to it, and a separate procedure, DisposeStr, is needed to manage memory properly.

Step 2: Update C# Import

Next, update the C# code to reflect the changes in the Delphi function signatures. Instead of returning strings directly, the C# code will deal with pointers.

Updated C# Code

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In this C# code:

Calling TestWide retrieves a pointer to the returned string.

Using Marshal.PtrToStringUni converts the pointer back to a managed string.

Memory Management: DisposeStr is called to free the allocated memory after usage.

Conclusion

Returning strings of unknown length from a Delphi DLL to a C# API can be tricky, but with the right management of pointers and memory, it becomes a manageable task. By modifying both the Delphi and C# code, you can effectively handle string data interchange without encountering runtime errors.

With the methods outlined in this post, you should be well-equipped to implement string handling in your applications, allowing for flexible and robust interoperability between Delphi and C# .

Feel free to share your experiences or additional tips in the comments below!