How to Fix the .NET 10 InlineArray TypeLoadException: Step-by-Step Solutions

With the release of .NET 10, developers gained access to enhanced value-type helpers designed to make stack-allocated, fixed-size buffers safer and more efficient. Among these features, the InlineArrayAttribute offers a modern alternative to the traditional fixed keyword. However, a compatibility change and compiler mismatch in .NET 10 can trigger an unexpected TypeLoadException at runtime when an explicit size is specified.

Understanding the .NET 10 InlineArray TypeLoadException

The issue typically manifests when a project compiles successfully but fails immediately upon execution. When the runtime attempts to load a value type that combines InlineArrayAttribute with an explicit size, such as StructLayoutAttribute.Size, it throws a TypeLoadException. This exception indicates that the type has an invalid layout or that a field size does not match the declared inline array size.

System.TypeLoadException: Could not load type ‘MyNamespace.MyVector’ because it contains an object field at offset 0 that is not a reference type.

The Root Cause: Compiler and Runtime Mismatch

The error stems from a temporary regression in how the Roslyn compiler and the Common Language Runtime (CLR) interact. When processing an explicit size declaration like [InlineArray(8)], the compiler is expected to emit a hidden fixed-size buffer field of size N * sizeof(T). Instead, the compiler emits the hidden field as a single element of type T, while still attaching a FixedBufferAttribute with the full length.

When the CLR attempts to load this metadata, it validates the layout. Because the actual field size does not match the expected total size, the validator rejects the type, resulting in a runtime failure before any user code executes.

Effective Workarounds to Resolve the Exception

Fortunately, developers can resolve this issue using several reliable strategies while waiting for official runtime patches to be applied.

1. The Wrapper Struct Pattern

The most robust workaround is to encapsulate the inline array within a non-generic public wrapper struct. By keeping the actual inline array internal or private, the public-facing type avoids triggering the faulty CLR layout validation path.

using System.Runtime.CompilerServices;

internal struct RawInlineArray
{
    [InlineArray(4)]
    public int _element;
}

public readonly struct SafeVector
{
    private readonly RawInlineArray _raw;

    public int this[int index]
    {
        get => Unsafe.Add(ref Unsafe.As<RawInlineArray, int>(ref Unsafe.AsRef(in _raw)), index);
    }
}

This approach maintains high performance because the Just-In-Time (JIT) compiler can inline the Unsafe.Add operations, ensuring minimal overhead.

2. Omitting the Explicit Size

Another alternative is to let the compiler infer the size directly from the field type using standard C# fixed-size buffer syntax. This avoids the InlineArrayAttribute explicit size path entirely:

public struct MyVector
{
    public int _fixedBuffer[4];
}

3. Utilizing the Generic Helper

For teams utilizing the latest updates, .NET 10 introduces a generic helper System.Runtime.CompilerServices.InlineArray<T, TSize>. This struct can be used as a field without needing the explicit attribute, bypassing the validation bug.

Tracking the Resolution

This behavior is tracked under official repository issues, specifically dotnet/runtime #106543 and dotnet/roslyn #10287. The fix is integrated into the .NET 10.0.1 release. For projects locked to earlier release candidates, implementing the wrapper pattern remains the safest and most compatible path forward.