dotnet/wpf · error · ArgumentNullException

paragraphProperties.DefaultTextRunProperties

Error message

paragraphProperties.DefaultTextRunProperties

What it means

TextFormatterImp.VerifyTextFormattingArguments validates arguments to FormatLine/FormatParagraph. It throws ArgumentNullException when TextParagraphProperties.DefaultTextRunProperties is null. The library requires default run properties on the paragraph properties because they supply the typeface and font metrics used for line formatting.

Solutions

Set DefaultTextRunProperties to a valid TextRunProperties instance (e.g. a new TextRunProperties or SimpleTextProps) in your TextParagraphProperties implementation
Check your custom TextParagraphProperties constructor for a missing DefaultTextRunProperties assignment
Throw your own ArgumentNullException in your paragraph-properties constructor to catch this at construction time

Example fix

// before
public class MyParaProps : TextParagraphProperties { /* never set DefaultTextRunProperties */ }
// after
public class MyParaProps : TextParagraphProperties {
    public MyParaProps() {
        DefaultTextRunProperties = new SimpleTextProps(typeface, 12.0); // must be non-null
    }
}

Defensive patterns

Strategy: validation

Validate before calling

if (paragraphProperties == null) throw new ArgumentNullException(nameof(paragraphProperties));
if (paragraphProperties.DefaultTextRunProperties == null) throw new ArgumentNullException("DefaultTextRunProperties");

Type guard

bool IsValidParaProps(TextParagraphProperties p) => p?.DefaultTextRunProperties != null;

Try / catch

try { formatter.FormatLine(...); } catch (ArgumentNullException) { /* supply valid DefaultTextRunProperties and retry */ }

Prevention

Always initialize DefaultTextRunProperties in TextParagraphProperties constructors
Assert non-null DefaultTextRunProperties before calling TextFormatter APIs

When it happens

Trigger: Calling TextFormatter.FormatLine (via PrepareFormatSettings) with a TextParagraphProperties subclass whose DefaultTextRunProperties was never set (left null in the constructor or manually assigned null).

Common situations: Custom TextParagraphProperties implementations where the constructor forgot to initialize DefaultTextRunProperties; refactoring that removed the TextRunProperties assignment; passing a default-constructed paragraph properties object.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/809e1ab503190573. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/TextFormatting/TextFormatterImp.cs:451

        /// <summary>
        /// Verify all text formatting arguments
        /// </summary>
        private void VerifyTextFormattingArguments(
            TextSource                  textSource,
            int                         firstCharIndex,
            double                      paragraphWidth,
            TextParagraphProperties     paragraphProperties,
            TextRunCache                textRunCache
            )
        {
            ArgumentNullException.ThrowIfNull(textSource);

            ArgumentNullException.ThrowIfNull(textRunCache);

            ArgumentNullException.ThrowIfNull(paragraphProperties);

            if (paragraphProperties.DefaultTextRunProperties == null)
                throw new ArgumentNullException("paragraphProperties.DefaultTextRunProperties");

            if (paragraphProperties.DefaultTextRunProperties.Typeface == null)
                throw new ArgumentNullException("paragraphProperties.DefaultTextRunProperties.Typeface");

            ArgumentOutOfRangeException.ThrowIfEqual(paragraphWidth, double.NaN);
            ArgumentOutOfRangeException.ThrowIfNegative(paragraphWidth);
            ArgumentOutOfRangeException.ThrowIfEqual(paragraphWidth, double.PositiveInfinity);
            ArgumentOutOfRangeException.ThrowIfGreaterThan(paragraphWidth, Constants.RealInfiniteWidth);

            double realMaxFontRenderingEmSize = Constants.RealInfiniteWidth / Constants.GreatestMutiplierOfEm;

            ArgumentOutOfRangeException.ThrowIfNegative(paragraphProperties.DefaultTextRunProperties.FontRenderingEmSize, "paragraphProperties.DefaultTextRunProperties.FontRenderingEmSize");
            ArgumentOutOfRangeException.ThrowIfGreaterThan(paragraphProperties.DefaultTextRunProperties.FontRenderingEmSize, realMaxFontRenderingEmSize, "paragraphProperties.DefaultTextRunProperties.FontRenderingEmSize");
            ArgumentOutOfRangeException.ThrowIfGreaterThan(paragraphProperties.Indent, Constants.RealInfiniteWidth, "paragraphProperties.Indent");
            ArgumentOutOfRangeException.ThrowIfGreaterThan(paragraphProperties.LineHeight, Constants.RealInfiniteWidth, "paragraphProperties.LineHeight");
            ArgumentOutOfRangeException.ThrowIfNegative(paragraphProperties.DefaultIncrementalTab, "paragraphProperties.DefaultIncrementalTab");
            ArgumentOutOfRangeException.ThrowIfGreaterThan(paragraphProperties.DefaultIncrementalTab, Constants.RealInfiniteWidth, "paragraphProperties.DefaultIncrementalTab");
        }

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