Скачать или смотреть Resolving Access Violation Errors with TBitmap in DataSnap ISAPI DLLs

Learn how to address the `Access Violation` error in TBitmap when using DataSnap ISAPI DLLs in RAD Studio by utilizing the TJPEGImage class instead.
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This video is based on the question https://stackoverflow.com/q/62996614/ asked by the user 'Moj.H' ( https://stackoverflow.com/u/8740033/ ) and on the answer https://stackoverflow.com/a/63020358/ provided by the user 'Moj.H' ( https://stackoverflow.com/u/8740033/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving Access Violation Errors with TBitmap in DataSnap ISAPI DLLs

When developing a DataSnap REST server application using RAD Studio 10.3.2, you might encounter an unexpected Access Violation error. This error can be particularly frustrating, especially if your application runs smoothly outside of the ISAPI DLLs. In this guide, we will uncover the root cause of this issue and present a reliable solution to ensure your image processing works seamlessly in your server methods.

Understanding the Problem

In a typical workflow, your server method receives an image from a client application as a base64 encoded string within a JSON object. You attempt to decode this image and load it into a TBitmap object. However, as many developers have discovered, when running this as an ISAPI DLL, the call to LoadFromStream results in an Access Violation error. The following code snippet exemplifies the scenario:

[[See Video to Reveal this Text or Code Snippet]]

Why Does This Happen?

The core issue arises from incompatibilities between the FMX.Graphics.TBitmap class and the ISAPI DLL environment. It is known that certain components from FireMonkey do not operate reliably when compiled as DLLs for web servers. This discrepancy can lead to access violations and other unpredictable behaviors.

The Solution

To overcome these challenges, you can switch from using the FMX.Graphics.TBitmap to the VCL.Imaging.jpeg.TJPEGImage class. The TJPEGImage class is more stable in the context of ISAPI applications.

Step-by-Step Resolution

Update the Uses Clause: Begin by adding the necessary unit for JPEG handling to the uses section:

[[See Video to Reveal this Text or Code Snippet]]

Modify Your Function: Update your getImage function to utilize TJPEGImage. Here’s the revised code:

[[See Video to Reveal this Text or Code Snippet]]

Verifying Your Changes

After implementing the changes, it’s essential to test your application thoroughly. Check if the encoding and decoding of the image works correctly and confirm that the application does not encounter any access violations during operation in the ISAPI context.

Conclusion

By substituting the FMX.Graphics.TBitmap class with the VCL.Imaging.jpeg.TJPEGImage class, we eliminated the Access Violation error when using ISAPI DLLs. This simple yet effective change allows for smooth handling of images in your DataSnap REST server application.

In summary, always consider the compatibility of the components you're using, especially in a web server or DLL context. If issues arise, check if there are alternative classes that can serve the same purpose effectively.