dotnet/wpf · error · NotSupportedException

SR.WriteNotSupported

Error message

SR.WriteNotSupported

What it means

VerifyStreamWriteArgs throws NotSupportedException when the stream does not support writing (CanWrite is false). WPF packaging streams (e.g. streams opened from read-only package parts) are often read-only, so any Write call fails before argument checks run.

Solutions

  1. Check stream.CanWrite before writing; if false, open the package/part in write mode (e.g. FileAccess.ReadWrite or FileMode.OpenOrCreate).
  2. Write to a temporary copy of the package, then replace the original if the source must stay read-only.
  3. Restructure code so writes go through Package/part APIs that open writable streams.

Example fix

// before
using var part = package.GetPart(partUri);
part.GetStream().Write(data, 0, data.Length); // read-only stream
// after
using var part = package.CreatePart(partUri, contentType, CompressionOption.Maximum);
using var s = part.GetStream(FileMode.Create, FileAccess.Write);
s.Write(data, 0, data.Length);
Defensive patterns

Strategy: validation

Validate before calling

if (!stream.CanWrite)
    throw new InvalidOperationException("Stream is read-only; open the package part with FileAccess.Write.");

Type guard

static bool IsWritable(Stream s) => s != null && s.CanWrite;

Try / catch

try { stream.Write(data, 0, data.Length); }
catch (NotSupportedException) { /* open part in write mode or copy package to temp and edit */ }

Prevention

When it happens

Trigger: Calling Write (or a wrapper using VerifyStreamWriteArgs) on a stream obtained from a package part opened in read mode, or a stream whose CanWrite returned false.

Common situations: Opening a ZipPackage part for reading but attempting to write updated content; writing to a package inside a file opened FileAccess.Read; trying to modify a package embedded in a resource.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/0d51cd9608d630eb. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/Shared/MS/Internal/IO/Packaging/PackagingUtilities.cs:139

                if (offset + count > buffer.Length)
                {
                    throw new ArgumentException(SR.ReadBufferTooSmall, nameof(buffer));
                }
            }
        }

        /// <summary>
        /// VerifyStreamWriteArgs
        /// </summary>
        /// <param name="s"></param>
        /// <param name="buffer"></param>
        /// <param name="offset"></param>
        /// <param name="count"></param>
        /// <remarks>common argument verification for Stream.Write</remarks>
        internal static void VerifyStreamWriteArgs(Stream s, byte[] buffer, int offset, int count)
        {
            if (!s.CanWrite)
                throw new NotSupportedException(SR.WriteNotSupported);

            ArgumentNullException.ThrowIfNull(buffer);

            if (offset < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(offset), SR.OffsetNegative);
            }

            if (count < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(count), SR.WriteCountNegative);
            }

            checked
            {
                if (offset + count > buffer.Length)
                    throw new ArgumentException(SR.WriteBufferTooSmall, nameof(buffer));
            }

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