dotnet/wpf · error · ArgumentException

SR.Image_InvalidArrayForPixel

Error message

SR.Image_InvalidArrayForPixel

What it means

The generic WritePixels(Array, ...) overload accepts arrays of arbitrary value types (byte[], int[], PixelColor[], etc.) but rejects arrays of reference types, since pixels are copied bitwise from array memory. If the array's element type is null or not a value type, it throws ArgumentException with SR.Image_InvalidArrayForPixel.

Solutions

1. Use a struct array (e.g., byte[], uint[], or a struct with LayoutKind.Sequential) instead of class-typed arrays.
2. Define pixel representation as a struct with explicit layout matching the pixel format.
3. Convert class-based pixel data into a flat value-type buffer before calling WritePixels.

Example fix

// before
class Pixel { public byte B, G, R, A; }
var px = new Pixel[w*h];
bitmap.WritePixels(rect, px, stride, 0); // throws
// after
struct Pixel { public byte B, G, R, A; }
var px = new Pixel[w*h];
bitmap.WritePixels(rect, px, stride, 0);

bool ok = sourceBuffer != null && sourceBuffer.GetType().GetElementType()?.IsValueType == true;

bool IsValueTypeArray<T>(T[] buf) where T : struct => buf != null; // constrain generics: void WritePixels<T>(Rect r, T[] buf, ...) where T : struct

try { bitmap.WritePixels(rect, buf, stride, 0); } catch (ArgumentException e) when (e.Message.Contains("pixel")) { /* convert to value-type buffer */ }

When it happens

Trigger: Calling WritePixels with a class-typed array such as object[], string[], or a struct array boxed incorrectly — the resolved elementType is not IsValueType.

Common situations: Passing arrays of custom classes that look like pixel structs but are reference types; passing object[] holding ints; reflection-built arrays with unknowable element type.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• ArgumentOutOfRangeException (timeout was Duration.Automatic)
• SR.Image_InsufficientBuffer
• SR.Image_InsufficientBufferSize
• ArgumentNullException (buffer/sourceBuffer was IntPtr.Zero)
• Collection_BadRank

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/8b95ad0e91269b81. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Imaging/WriteableBitmap.cs:392

            Int32Rect sourceRect,
            Array     sourceBuffer,
            int       sourceBufferStride,
            int       destinationX,
            int       destinationY
            )
        {
            WritePreamble();

            ValidateArrayAndGetInfo(sourceBuffer,
                                    backwardsCompat: false,
                                    out _,
                                    out uint sourceBufferSize,
                                    out Type elementType);

            // We accept arrays of arbitrary value types - but not reference types.
            if (elementType == null || !elementType.IsValueType)
            {
                throw new ArgumentException(SR.Image_InvalidArrayForPixel);
            }

            // Get the address of the data in the array by pinning it.
            unsafe
            {
                fixed (byte* buffer = &MemoryMarshal.GetArrayDataReference(sourceBuffer))
                    WritePixelsImpl(sourceRect,
                                    (nint)buffer,
                                    sourceBufferSize,
                                    sourceBufferStride,
                                    destinationX,
                                    destinationY,
                                    backwardsCompat: false);
            }
        }

        /// <summary>
        /// Update the pixels of this Bitmap

View on GitHub (pinned to 81131a70a4)