spectreconsole/spectre.console · error · ArgumentNullException

Value cannot be null. (Parameter 'writer')

Error message

Value cannot be null. (Parameter 'writer')

What it means

Thrown by the AnsiConsoleOutput constructor when the supplied TextWriter is null. AnsiConsoleOutput is immutable once built, so the writer must be provided up front; a null writer would make every later write fail, so it is rejected immediately as an ArgumentNullException.

Source

Thrown at src/Spectre.Console/AnsiConsoleOutput.cs:42

            }

            return false;
        }
    }

    /// <inheritdoc/>
    public int Width => ConsoleHelper.GetSafeWidth();

    /// <inheritdoc/>
    public int Height => ConsoleHelper.GetSafeHeight();

    /// <summary>
    /// Initializes a new instance of the <see cref="AnsiConsoleOutput"/> class.
    /// </summary>
    /// <param name="writer">The output writer.</param>
    public AnsiConsoleOutput(TextWriter writer)
    {
        Writer = writer ?? throw new ArgumentNullException(nameof(writer));
    }

    /// <inheritdoc/>
    public void SetEncoding(Encoding encoding)
    {
        if (Writer.IsStandardOut() || Writer.IsStandardError())
        {
            System.Console.OutputEncoding = encoding;
        }
    }
}

View on GitHub (pinned to 0acc92fada)

Solutions

1. Pass a concrete TextWriter such as Console.Out, Console.Error, or a new StringWriter()
2. Initialize the writer variable before constructing AnsiConsoleOutput
3. Use a nullable annotation / null check at the call site if the writer source may be absent

Example fix

// before
TextWriter w = null;
var output = new AnsiConsoleOutput(w); // throws

// after
var output = new AnsiConsoleOutput(Console.Out);

if (writer is null)
    writer = Console.Out;
var output = new AnsiConsoleOutput(writer);

static bool IsUsableWriter(TextWriter? w) => w is not null;

try
{
    var output = new AnsiConsoleOutput(writer);
}
catch (ArgumentNullException ex) when (ex.ParamName == "writer")
{
    writer = Console.Out;
    var output = new AnsiConsoleOutput(writer);
}

When it happens

Trigger: Calling new AnsiConsoleOutput(null) or otherwise constructing it with a null TextWriter reference.

Common situations: Passing a writer field that was never initialized; a DI/lookup that returns null for the configured output; refactoring that drops the Console.Out argument; test helper that forgot to supply a StringWriter.

Related errors

• Output writer was null
• Value cannot be null. (Parameter 'obj')
• Cannot convert color to console color.
• Profile enricher of type '{enricher.GetType().FullName}' doe
• Value cannot be null. (Parameter 'value')

AI-assisted analysis of spectreconsole/spectre.console@0acc92fada (2026-08-13). Data as JSON: /api/errors/d22aed76f774a3f8. Report an issue: GitHub.