dotnet/wpf · error · NullReferenceException

Buffer address passed to GetText cannot be NULL.

Error message

Buffer address passed to GetText cannot be NULL.

What it means

IFilter.GetText rejects a null (IntPtr.Zero) buffer before attempting to write chunk text into unmanaged memory. The COM/IFilter contract requires the caller (the index service) to supply a valid pre-allocated character buffer sized by bufCharacterCount; a zero pointer means the caller violated that contract, so the filter throws NullReferenceException rather than corrupting memory via Marshal writes.

Solutions

Allocate a valid buffer (Marshal.AllocHGlobal) sized bufCharacterCount * sizeof(char) before calling GetText
Check buffer allocation success before calling GetText
If the count is zero GetText is a no-op, but never pass IntPtr.Zero with a nonzero count

Example fix

// before
filter.GetText(ref count, IntPtr.Zero);
// after
IntPtr buffer = Marshal.AllocHGlobal((int)(count * 2));
try { filter.GetText(ref count, buffer); }
finally { Marshal.FreeHGlobal(buffer); }

Defensive patterns

Strategy: validation

Validate before calling

if (pBuffer == IntPtr.Zero && bufCharacterCount > 0)
    throw new ArgumentException("GetText buffer must be allocated", nameof(pBuffer));

Try / catch

try { filter.GetText(ref count, buffer); }
catch (NullReferenceException) { /* reallocate buffer and retry */ }

Prevention

Always allocate unmanaged buffers before interop calls
Never use IntPtr.Zero to probe required buffer sizes
Free buffers in finally blocks to avoid cascading allocation failures

When it happens

Trigger: Calling IFilter.GetText with pBuffer == IntPtr.Zero and a non-zero bufCharacterCount.

Common situations: Interop callers whose Marshal.AllocHGlobal allocation failed or returned IntPtr.Zero, or callers mistakenly passing IntPtr.Zero to probe required buffer size.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/420f72b41edbf454. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/IO/Packaging/XpsFilter.cs:111

        /// <summary>
        /// Gets text content corresponding to current chunk.
        /// </summary>
        /// <param name="bufCharacterCount">size of buffer in characters</param>
        /// <param name="pBuffer">buffer pointer</param>
        /// <remarks>Supported for indexing content of Package.</remarks>
        void IFilter.GetText(ref uint bufCharacterCount, IntPtr pBuffer)
        {
            if (_filter == null)
            {
                throw new COMException(SR.FileToFilterNotLoaded,
                    (int)FilterErrorCode.FILTER_E_ACCESS);
            }

            // NULL is not an acceptable value for pBuffer
            if (pBuffer == IntPtr.Zero)
            {
                throw new NullReferenceException(SR.FilterNullGetTextBufferPointer);
            }

            // If there is 0 byte to write, this is a no-op.
            if (bufCharacterCount == 0)
            {
                return;
            }

            // Because we should always return the string with null terminator, a buffer size
            // of one character can hold the null terminator only, we can always write the 
            // terminator to the buffer and return directly.
            if (bufCharacterCount == 1)
            {
                Marshal.WriteInt16(pBuffer, 0);
                return;
            }

            // Record the original buffer size. bufCharacterCount may be changed later.

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