PrismLibrary/Prism · error · ModularityException

Resources.DependencyOnMissingModule (formatted with…

Error message

Resources.DependencyOnMissingModule (formatted with moduleNames)

What it means

This ModularityException is thrown by ModuleDependencySolver.Solve when a module in the graph depends on a module that was never registered with AddModule. The message lists the missing module names, produced by FindMissingModules.

Solutions

Add the missing module to the catalog/solver with AddModule before solving
Remove the stale dependency from the depending module's declaration
Check the exception message (it contains the missing names) and align the spellings exactly

Example fix

// before
// ModuleInfo for "CoreModule" missing from catalog, but:
[Module(ModuleName="FeatureA", DependsOn=new[]{"CoreModule"})]
// after
// ensure CoreModule's ModuleInfo is added to the ModuleCatalog

Defensive patterns

Strategy: try-catch

Validate before calling

var missing = deps.Keys
    .SelectMany(k => deps[k])
    .Where(dep => !knownModules.Contains(dep))
    .ToList();
if (missing.Count > 0)
    throw new InvalidOperationException($"Missing modules: {string.Join(",", missing)}");

Try / catch

try
{
    var order = solver.Solve();
}
catch (ModularityException ex)
{
    logger.LogError(ex, "Modules depend on missing modules: {0}", ex.Message);
}

Prevention

Keep ModuleCatalog entries and DependsOn declarations in sync
Run Solve in integration tests before shipping
Use module-name constants shared between declarations and consumers

When it happens

Trigger: Calling AddDependency("A", "B") where "B" is not in knownModules — e.g. a module in config lists a dependency on a module not present in the catalog; case mismatch between the two name usages.

Common situations: Removing a module from the catalog while other modules still declare a dependency on it; renaming a module but forgetting to update consumers' DependsOn attributes; loading a partial module set in tests.

Understand the failure class

Background: "not installed", "pip install", "required for": how missing-dependency errors surface across open-source libraries — this error's family across 34 libraries.

Related errors

AI-assisted analysis of PrismLibrary/Prism@358118cd64 (2026-09-15). Data as JSON: /api/errors/e28ea65d3bab42a1. Report an issue: GitHub.

Appendix: source

Thrown at src/Prism.Core/Modularity/ModuleDependencySolver.cs:93

        /// when a cycle is found in the defined dependency graph.</exception>
        public string[] Solve()
        {
            List<string> skip = new List<string>();
            while (skip.Count < dependencyMatrix.Count)
            {
                List<string> leaves = this.FindLeaves(skip);
                if (leaves.Count == 0 && skip.Count < dependencyMatrix.Count)
                {
                    throw new CyclicDependencyFoundException(Resources.CyclicDependencyFound);
                }
                skip.AddRange(leaves);
            }
            skip.Reverse();

            if (skip.Count > knownModules.Count)
            {
                string moduleNames = this.FindMissingModules(skip);
                throw new ModularityException(moduleNames, string.Format(CultureInfo.CurrentCulture,
                                                            Resources.DependencyOnMissingModule,
                                                            moduleNames));
            }

            return skip.ToArray();
        }

        private string FindMissingModules(List<string> skip)
        {
            string missingModules = "";

            foreach (string module in skip)
            {
                if (!knownModules.Contains(module))
                {
                    missingModules += ", ";
                    missingModules += module;
                }

View on GitHub (pinned to 358118cd64)