{"record":{"id":"dab5dcbb5ec72f61","repo":"louthy/language-ext","slug":"constructor-not-found-for-type-typeof-r-fullname","errorCode":null,"errorMessage":"Constructor not found for type {typeof(R).FullName}","messagePattern":"Constructor not found for type (.+?)","errorType":"exception","errorClass":"ArgumentException","httpStatus":null,"severity":"error","filePath":"LanguageExt.Core/Utility/IL.cs","lineNumber":24,"sourceCode":"using System.Reflection.Emit;\nusing System.Runtime.Serialization;\nusing System.Text;\nusing LanguageExt.Traits.Resolve;\nusing static LanguageExt.Prelude;\nusing static LanguageExt.Reflect;\n\nnamespace LanguageExt;\n\npublic static class IL\n{\n    /// <summary>\n    /// Emits the IL to instantiate a type of R with a single argument to \n    /// the constructor\n    /// </summary>\n    public static Func<R> Ctor<R>()\n    {\n        var ctorInfo = GetConstructor<R>()\n           .IfNone(() => throw new ArgumentException($\"Constructor not found for type {typeof(R).FullName}\"));\n\n        var dynamic = new DynamicMethod(\"CreateInstance\",\n                                        ctorInfo.DeclaringType,\n                                        Type.EmptyTypes,\n                                        typeof(R).Module,\n                                        true);\n\n        var il = dynamic.GetILGenerator();\n        il.Emit(OpCodes.Newobj, ctorInfo);\n        il.Emit(OpCodes.Ret);\n\n        return (Func<R>)dynamic.CreateDelegate(typeof(Func<R>));\n    }\n\n    /// <summary>\n    /// Emits the IL to instantiate a type of R with a single argument to \n    /// the constructor\n    /// </summary>","sourceCodeStart":6,"sourceCodeEnd":42,"githubUrl":"https://github.com/louthy/language-ext/blob/2f0e3628242889774d4141960a35671a0280051f/LanguageExt.Core/Utility/IL.cs#L6-L42","documentation":"IL.Ctor<R>() builds a compiled delegate that instantiates R via its parameterless constructor, emitting DynamicMethod IL for speed. Before emitting, it looks up the constructor with GetConstructor<R>(); if the type has no accessible parameterless constructor, it throws ArgumentException with the type's FullName. This library throws it because IL emission requires a concrete ConstructorInfo to generate the call.","triggerScenarios":"Calling IL.Ctor<R>() where typeof(R) has no public parameterless constructor — e.g. R is a record/class whose only constructors take parameters, R is an interface or abstract class, R is a struct with no explicit default ctor path matching the lookup, or the matching ctor is non-public.","commonSituations":"Refactoring a type used with IL.Ctor to add constructor parameters without updating call sites; mapping via LanguageExt utilities over types with required arguments; using types from assemblies trimmed of ctors; accidentally pointing Ctor<T>() at an interface or abstract base type.","solutions":["Add a public parameterless constructor to R, or call the IL.Ctor overload matching the actual constructor arity (e.g. IL.Ctor<A, R> for one argument).","If R cannot be changed, switch to Activator.CreateInstance<R>() or a factory lambda () => new R(...) instead of IL emission.","Verify R is a concrete, non-abstract, non-interface type with an accessible (public) parameterless constructor before calling IL.Ctor<R>()."],"exampleFix":"// before\nvar make = IL.Ctor<Person>(); // Person has no parameterless ctor\n// after\nvar make = IL.Ctor<string, int, Person>(); // matches Person(string, int)","handlingStrategy":"validation","validationCode":"if (typeof(R).IsInterface || typeof(R).IsAbstract ||\n    R.GetConstructors().All(c => c.GetParameters().Length != 0))\n    throw new InvalidOperationException(\n        $\"{typeof(R).FullName} has no public parameterless constructor; cannot use IL.Ctor<R>()\");","typeGuard":"static bool HasParameterlessCtor<R>() =>\n    !typeof(R).IsInterface && !typeof(R).IsAbstract &&\n    typeof(R).GetConstructor(Type.EmptyTypes) != null;","tryCatchPattern":"Func<R> make;\ntry { make = IL.Ctor<R>(); }\ncatch (ArgumentException ex) when (ex.Message.Contains(\"Constructor not found\"))\n{\n    make = () => Activator.CreateInstance<R>(); // or another fallback\n}","preventionTips":["Only use IL.Ctor overloads whose arity matches a public constructor of the target type.","Add unit tests that resolve the IL.Ctor delegate for every type used with it.","Never point IL.Ctor at interfaces, abstract classes, or types with non-public constructors.","Keep constructor signatures and IL.Ctor call sites in sync during refactors (search for IL.Ctor before changing ctors)."],"tags":["reflection","il-emission","constructor","generics"],"backgroundTag":"constructor-not-found","analyzedSha":"2f0e3628242889774d4141960a35671a0280051f","analyzedAt":"2026-09-15T03:31:55.716Z","contentChangedAt":"2026-09-15T03:31:55.716Z","schemaVersion":2},"datasetVersion":"2026-09-16T09:17:16.951Z"}