What causes "SR.FamilyMap_TargetNotSet" in dotnet/wpf?

Set familyMap.Target (e.g. to a font family name like "Segoe UI") before adding the map.

dotnet/wpf · error · ArgumentException

SR.FamilyMap_TargetNotSet

Error message

SR.FamilyMap_TargetNotSet

What it means

CompositeFontInfo.PrepareToAddFamilyMap validates a FontFamilyMap before adding it to a composite font (font family fallback) definition. A family map must have a non-null, non-empty Target (the target font family name); otherwise it throws ArgumentException(SR.FamilyMap_TargetNotSet).

Solutions

Set familyMap.Target (e.g. to a font family name like "Segoe UI") before adding the map.
Fix the composite font XAML so every <FontFamilyMap> has a Target attribute.
Validate Target non-empty in your own code before invoking the add operation.

Example fix

// before
var map = new FontFamilyMap { Language = XmlLanguage.GetLanguage("ja-JP") };
compositeFont.PrepareToAddFamilyMap(map); // throws: Target not set
// after
var map = new FontFamilyMap { Language = XmlLanguage.GetLanguage("ja-JP"), Target = "Yu Gothic" };
compositeFont.PrepareToAddFamilyMap(map);

Defensive patterns

Strategy: validation

Validate before calling

if (familyMap == null) throw new ArgumentNullException(nameof(familyMap));
if (string.IsNullOrEmpty(familyMap.Target))
    throw new ArgumentException("FontFamilyMap.Target must be set before adding.");

Type guard

bool IsValidFamilyMap(FontFamilyMap m) => m != null && !string.IsNullOrEmpty(m.Target);

Try / catch

try { compositeFont.PrepareToAddFamilyMap(map); }
catch (ArgumentException) { /* fix or skip the family map */ }

Prevention

Always set Target on FontFamilyMap before adding.
Validate composite font XAML: every FontFamilyMap needs Target.
Check hand-edited CompositeFont resources for missing attributes.

When it happens

Trigger: Adding a FontFamilyMap to a CompositeFontInfo where familyMap.Target was never assigned — e.g. constructing a FontFamilyMap in code and calling the add API before setting Target, or parsing a malformed composite font where the FontFamily element's Target attribute is missing.

Common situations: Programmatic composition of a font fallback list; hand-written or corrupted .xaml/CompositeFont resources missing the Target attribute for a FamilyMap entry; XAML parsers registering family maps during CompositeFont loading.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/1fb09b144a2bf3bf. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/FontFace/CompositeFontInfo.cs:48

        /// Construct a composite font
        /// </summary>
        internal CompositeFontInfo()
        {
            _familyNames = new LanguageSpecificStringDictionary(new Dictionary<XmlLanguage,string>(InitialCultureCount));
            _familyMaps = new FontFamilyMapCollection(this);
            _defaultFamilyMapRanges = EmptyFamilyMapRanges;
        }

        /// <summary>
        /// Called by FontFamilyMapCollection when a FontFamilyMap is being added.
        /// </summary>
        internal void PrepareToAddFamilyMap(FontFamilyMap familyMap)
        {
            // Validate parameters.
            ArgumentNullException.ThrowIfNull(familyMap);

            if (string.IsNullOrEmpty(familyMap.Target))
                throw new ArgumentException(SR.FamilyMap_TargetNotSet);

            // If it's culture-specific make sure it's in the hash table.
            if (familyMap.Language != null)
            {
                if (_familyMapRangesByLanguage == null)
                {
                    _familyMapRangesByLanguage = new Dictionary<XmlLanguage, ushort[]>(InitialCultureCount);
                    _familyMapRangesByLanguage.Add(familyMap.Language, EmptyFamilyMapRanges);
                }
                else if (!_familyMapRangesByLanguage.ContainsKey(familyMap.Language))
                {
                    _familyMapRangesByLanguage.Add(familyMap.Language, EmptyFamilyMapRanges);
                }
            }
        }

        #region family map ranges (skip lists)

View on GitHub (pinned to 81131a70a4)