antlr/antlr4 · error · NotSupportedException

Unbuffered stream cannot know its size

Error message

Unbuffered stream cannot know its size

What it means

UnbufferedCharStream deliberately does not track total input length because it has not necessarily read the whole source. Its Size property therefore always throws NotSupportedException. Code that asks for the size of an ICharStream must be prepared for streams that cannot know it.

Source

Thrown at runtime/CSharp/src/UnbufferedCharStream.cs:410

                }
            }
            p = i;
            currentCharIndex = index;
            if (p == 0)
            {
                lastChar = lastCharBufferStart;
            }
            else
            {
                lastChar = data[p - 1];
            }
        }

        public virtual int Size
        {
            get
            {
                throw new NotSupportedException("Unbuffered stream cannot know its size");
            }
        }

        public virtual string SourceName
        {
            get
            {
                if (string.IsNullOrEmpty(name))
                {
                    return IntStreamConstants.UnknownSourceName;
                }
                return name;
            }
        }

        public virtual string GetText(Interval interval)
        {
            if (interval.a < 0 || interval.b < interval.a - 1)

View on GitHub (pinned to 7d5770395b)

Solutions

1. Do not read Size from UnbufferedCharStream.
2. Use stream.Index for the current absolute position when appropriate.
3. Use AntlrInputStream if input can be fully buffered and Size is required.
4. Compute total length separately from the source if absolutely necessary.

Example fix

// before
int size = unbuffered.Size;

// after
var buffered = new AntlrInputStream(reader);
int size = buffered.Size;

if (stream is UnbufferedCharStream)
    throw new InvalidOperationException("Size is unavailable; buffer the input first.");

static bool HasKnownSize(ICharStream stream) => stream is not UnbufferedCharStream;

try { int size = stream.Size; }
catch (NotSupportedException ex) when (ex.Message == "Unbuffered stream cannot know its size") { /* use stream.Index or AntlrInputStream */ }

When it happens

Trigger: Accessing stream.Size on UnbufferedCharStream, including logging, progress reporting, bounds checks, or generic algorithms that assume every character stream has a known length.

Common situations: Porting code from AntlrInputStream, which does expose Size; generic ICharStream utilities; or test harnesses that print input length before parsing.

Related errors

• cannot consume EOF
• release() called with an invalid marker.
• cannot seek to negative index ${index}
• seek to index outside buffer: ${index} not in ${bufferStartI
• interval ${interval} outside buffer: ${bufferStartIndex}..${

AI-assisted analysis of antlr/antlr4@7d5770395b (2026-08-14). Data as JSON: /api/errors/52bf97c563b75b6e. Report an issue: GitHub.