Improve exception message for empty assembly loading

[Copilot]

Description

Loading an empty assembly (0-length byte array or stream) throws BadImageFormatException with the generic message "Bad IL format." This is a common scenario with bugs in dynamically generated assemblies and provides no actionable information.

Changes

Before:

Assembly.Load(new byte[0]);  
// BadImageFormatException: Bad IL format.

After:

Assembly.Load(new byte[0]);
// BadImageFormatException: Assembly image is empty. The stream or byte array must contain a valid PE file.

Implementation

• Added BadImageFormat_EmptyAssembly resource string
• Updated Assembly.Load(byte[]) and AssemblyLoadContext.LoadFromStream() to use specific message when input is empty
• Updated tests to verify error message contains "empty"

The change is scoped to explicit empty checks in managed code. Native code paths (AssemblyNative_LoadFromStream) already assert non-empty input via managed validation.

cc @dotnet/appmodel @AaronRobinsonMSFT

Original prompt

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This section details on the original issue you should resolve

<issue_title>Improve exception message when loading an empty assembly</issue_title>
<issue_description>Based on dotnet/roslyn#39470, a relatively common issue when working with dynamically generated assemblies is that, due to a bug in your code, you attempt to load an empty assembly (e.g., you call Assembly.Load(Stream) with a Stream of Length 0). When this happens, you get a correct, but not particularly helpful exception:

System.BadImageFormatException: Bad IL format.

Would it make sense to improve the exception message, to make it more clear what the issue is?</issue_description>

Comments on the Issue (you are @copilot in this section)

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• Fixes Improve exception message when loading an empty assembly #36814

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[stephentoub]

@copilot please invoke the code-review skill and post the analysis/results as a comment on this PR

[Copilot]

@copilot please invoke the code-review skill and post the analysis/results as a comment on this PR

🤖 Copilot Code Review — PR #124517

Holistic Assessment

Motivation: ✅ Justified. Loading empty assemblies (0-byte streams/arrays) is a real scenario that users encounter when working with dynamically generated assemblies, and the current generic "Bad IL format" message provides no actionable guidance. The linked Roslyn issue confirms this is a genuine pain point.

Approach: ✅ Sound. The fix is surgical and targeted—it introduces a new, specific resource string for the empty assembly case while preserving the generic message for other BadImageFormat scenarios. The change is behavior-preserving (exception type and throwing conditions unchanged) and only improves diagnostic quality.

Summary: ✅ LGTM. This is a focused, low-risk improvement that makes an error message more actionable. The code changes are minimal, the tests validate the new behavior appropriately, and the approach follows established patterns for error message improvements in the codebase.

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Detailed Findings

✅ Core Implementation — Correctly Scoped

The change appropriately:

• Adds a new resource string (BadImageFormat_EmptyAssembly) rather than modifying the existing generic one (BadImageFormat_BadILFormat), preserving backward compatibility for other error scenarios
• Updates only the two explicit empty-input checks (rawAssembly.Length == 0 and spanAssembly.IsEmpty) to use the new message
• Leaves the generic message in place for the "stream too large" case (line 422 in AssemblyLoadContext.cs: if (((ulong)length) > (ulong)Array.MaxLength))

This is the correct approach—specific messages for diagnosable cases, generic messages for others.

✅ Test Quality — Appropriate Validation Strategy

The test assertions use Assert.Contains("empty", ex.Message, StringComparison.OrdinalIgnoreCase), which is the right pattern per dotnet/runtime conventions:

• Resilient to minor message wording changes
• Verifies the key diagnostic information is present
• Avoids brittle exact-string matching

Both empty-byte-array and empty-stream code paths are now covered by tests.

💡 Message Wording — Minor Clarity Consideration

The message reads: "Assembly image is empty. The stream or byte array must contain a valid PE file."

The second sentence could be read as "the problem is the PE file isn't valid" rather than "the problem is there's no data at all." Consider whether a slight rephrasing would be clearer, e.g.:

• "Assembly image is empty. The stream or byte array must contain data for a valid PE file."
• "Cannot load empty assembly. The stream or byte array must contain a valid PE file."

However, this is a minor point—the current message is substantially better than "Bad IL format" and clearly mentions "empty," which is the key diagnostic information. This is purely a suggestion for future refinement if desired.

✅ Cross-Cutting Consistency — Other Load Paths Considered

I verified whether file-based loading paths (Assembly.LoadFile) encounter the same issue. File-based loading goes through native code (PEImage::OpenImage), which will fail with a different error path if the file is 0 bytes (native CheckILFormat failure). The empty check is specific to in-memory byte arrays and streams, so limiting the change to those two call sites is correct.

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Verdict: This is a well-scoped, low-risk improvement that makes a common error scenario easier to diagnose. No blocking issues identified.

[Copilot]

Pull request overview

This PR improves the exception message when attempting to load an empty assembly (0-length byte array or stream). Previously, users received the generic "Bad IL format." message, which didn't provide actionable information for a common scenario when working with dynamically generated assemblies.

Changes:

• Added a new resource string BadImageFormat_EmptyAssembly with a descriptive error message
• Updated Assembly.Load(byte[]) and AssemblyLoadContext.LoadFromStream() to throw BadImageFormatException with the specific message when input is empty
• Added test coverage to verify the improved error message contains "empty"

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/Resources/Strings.resx	Added new resource string BadImageFormat_EmptyAssembly with descriptive message for empty assembly input
src/libraries/System.Private.CoreLib/src/System/Reflection/Assembly.cs	Updated Assembly.Load(byte[]) to use the new specific error message for empty byte arrays
src/libraries/System.Private.CoreLib/src/System/Runtime/Loader/AssemblyLoadContext.cs	Updated LoadFromStream() to use the new specific error message for empty streams
src/libraries/System.Runtime/tests/System.Reflection.Tests/AssemblyTests.cs	Updated test to verify the new error message contains "empty"
src/libraries/System.Runtime/tests/System.Reflection.Tests/CoreCLR/AssemblyTests.cs	Added new test to verify empty stream throws with correct error message