dotnet/efcore · error · ArgumentException

The property '{property}' of the argument '{argument}' canno

Error message

The property '{property}' of the argument '{argument}' cannot be 'null'.

What it means

Thrown in CSharpModelGenerator.GenerateModel (line 58) when ModelCodeGenerationOptions.ContextName is null. The scaffolding/reverse-engineering code generator needs a context class name to emit the DbContext file, so a null name is rejected with ArgumentException before any template runs. It is a contract violation on the options object passed to GenerateModel.

Source

Thrown at src/EFCore.Design/Scaffolding/Internal/CSharpModelGenerator.cs:58

    ///     any release. You should only use it directly in your code with extreme caution and knowing that
    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
    /// </summary>
    public override string Language
        => "C#";

    /// <summary>
    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
    ///     any release. You should only use it directly in your code with extreme caution and knowing that
    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
    /// </summary>
    public override ScaffoldedModel GenerateModel(
        IModel model,
        ModelCodeGenerationOptions options)
    {
        if (options.ContextName == null)
        {
            throw new ArgumentException(
                CoreStrings.ArgumentPropertyNull(nameof(options.ContextName), nameof(options)), nameof(options));
        }

        if (options.ConnectionString == null)
        {
            throw new ArgumentException(
                CoreStrings.ArgumentPropertyNull(nameof(options.ConnectionString), nameof(options)), nameof(options));
        }

        var host = new TextTemplatingEngineHost(_serviceProvider);
        var contextTemplate = new CSharpDbContextGenerator { Host = host, Session = host.CreateSession() };
        contextTemplate.Session.Add("Model", model);
        contextTemplate.Session.Add("Options", options);
        contextTemplate.Session.Add("NamespaceHint", options.ContextNamespace ?? options.ModelNamespace);
        contextTemplate.Session.Add("ProjectDefaultNamespace", options.RootNamespace);
        contextTemplate.Initialize();

        var generatedCode = ProcessTemplate(contextTemplate);

View on GitHub (pinned to dbf9771522)

Solutions

1. Set options.ContextName to a valid, non-null context class name before calling GenerateModel.
2. If scaffolding programmatically, derive a default name (e.g. from the database name) and assign it, mirroring ReverseEngineerScaffolder's behavior.
3. Validate all required ModelCodeGenerationOptions fields before invoking the generator.

Example fix

// before
var options = new ModelCodeGenerationOptions { ConnectionString = cs };
modelGenerator.GenerateModel(model, options); // throws: ContextName null

// after
var options = new ModelCodeGenerationOptions
{
    ConnectionString = cs,
    ContextName = "ShopContext"
};
modelGenerator.GenerateModel(model, options);

if (string.IsNullOrEmpty(options.ContextName))
    throw new InvalidOperationException("ContextName is required.");
// or set a default before calling the generator

try { generator.GenerateModel(model, options); }
catch (ArgumentException ex) when (ex.Message.Contains("ContextName"))
{ /* set options.ContextName and retry */ }

When it happens

Trigger: Calling GenerateModel (directly or via ScaffoldModel/ReverseEngineer) with options.ContextName left null. The check `if (options.ContextName == null)` fires and throws ArgumentException referencing CoreStrings.ArgumentPropertyNull(nameof(options.ContextName), nameof(options)).

Common situations: Programmatic scaffolding where the caller forgot to set ContextName on ModelCodeGenerationOptions. A wrapper/wizard that builds options incrementally and skips the name. Passing default(options)-style object. Note: ReverseEngineerScaffolder will derive a default ContextName, so this typically surfaces only when calling the code generator directly.

Related errors

• The context class name '{contextClassName}' is not a valid C
• Processing '{inputFile}' failed.
• Metadata model returned should not be 'null'. Provider: {pro
• You cannot add a migration with the name 'Migration'.
• The name '{migrationName}' is used by an existing migration.

AI-assisted analysis of dotnet/efcore@dbf9771522 (2026-08-06). Data as JSON: /api/errors/0832134b1aed8bb9. Report an issue: GitHub.