What causes "While scanning a simple key, could not find expected ':'." in stride3d/stride?

Add the missing ':' after the key on the reported line

stride3d/stride · error · SyntaxErrorException

While scanning a simple key, could not find expected ':'.

Error message

While scanning a simple key, could not find expected ':'.

What it means

The scanner saw a token that looked like a potential simple key (a scalar that could precede ':') but the ':' never arrived before a line break or the 1024-character limit. When that potential key is required (a mapping key in block context must have a colon), scanning fails with this SyntaxErrorException.

Solutions

Add the missing ':' after the key on the reported line
Ensure the key and its ':' are on the same line
If the scalar is a value not a key, fix indentation so it is not treated as a mapping key
Quote long or multi-line strings and use block scalars (| or >) for values

Example fix

// before
config:
  name
  value: 1
// after
config:
  name: myapp
  value: 1

Defensive patterns

Strategy: try-catch

Validate before calling

// Ensure every non-comment, non-list line in block context ends with ':' or contains ': '
foreach (var line in yamlText.Split('\n')) { var t = line.Trim(); if (t.Length>0 && !t.StartsWith("#") && !t.StartsWith("-") && !t.Contains(':')) throw new FormatException("Line lacks key/value colon: " + line); }

Try / catch

catch (SyntaxErrorException ex) { throw new ConfigFormatException($"Missing ':' near {ex.Start.Line}:{ex.Start.Column}"); }

Prevention

Never leave a bare key without a colon in a mapping
Keep key/value on a single line
Use block scalars (| or >) for multi-line strings
Lint YAML in CI before deployment

When it happens

Trigger: A block-context mapping line where the key scalar is followed by a line break instead of ':', e.g. 'key' alone on a line inside a block mapping, or an extremely long scalar (>1024 chars) with no colon.

Common situations: Forgot to type ':' after a key in a config file, a key wrapped onto the next line by copy/paste, or multiline plain scalars accidentally interpreted as keys.

Related errors

AI-assisted analysis of stride3d/stride@96fad776d2 (2026-09-14). Data as JSON: /api/errors/b46e088b717f6984. Report an issue: GitHub.

Appendix: source

Thrown at sources/core/Stride.Core.Yaml/Scanner.cs:273

        private void StaleSimpleKeys()
        {
            // Check for a potential simple key for each flow level.

            foreach (var key in simpleKeys)
            {
                // The specification requires that a simple key

                //  - is limited to a single line,
                //  - is shorter than 1024 characters.


                if (key.IsPossible && (key.Mark.Line < mark.Line || key.Mark.Index + 1024 < mark.Index))
                {
                    // Check if the potential simple key to be removed is required.

                    if (key.IsRequired)
                    {
                        throw new SyntaxErrorException(mark, mark, "While scanning a simple key, could not find expected ':'.");
                    }

                    key.IsPossible = false;
                }
            }
        }

        private void FetchNextToken()
        {
            // Check if we just started scanning.  Fetch STREAM-START then.

            if (!streamStartProduced)
            {
                FetchStreamStart();
                return;
            }

            // Eat whitespaces and comments until we reach the next token.

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