{"record":{"id":"b1ba59e3f69853b5","repo":"louthy/language-ext","slug":"append-method-found-for-stringbuilder","errorCode":null,"errorMessage":"Append method found for StringBuilder","messagePattern":"Append method found for StringBuilder","errorType":"exception","errorClass":"ArgumentException","httpStatus":null,"severity":"error","filePath":"LanguageExt.Core/Utility/IL.cs","lineNumber":780,"sourceCode":"                  }\n               .Concat( Fields().Bind(identity))\n               .Concat( new [] { Expression.Label(returnTarget, Zero) as Expression }));\n\n        var lambda = Expression.Lambda<Func<A, A, int>>(block, self, other);\n\n        return lambda.Compile();\n    }\n\n    static Func<A, string> ToStringExpr<A>(bool includeBase)\n    {\n        var fields = GetPublicInstanceFields<A>(\n            includeBase,\n            typeof(NonShowAttribute),\n            typeof(NonRecordAttribute)\n        ).ToArray();\n            \n        var stringBuilder = GetConstructor<StringBuilder>().IfNone(() => throw new ArgumentException($\"Constructor not found for StringBuilder\"));\n        var appendChar    = GetPublicInstanceMethod<StringBuilder, char>(\"Append\", true).IfNone(() => throw new ArgumentException($\"Append method found for StringBuilder\"));\n        var appendString  = GetPublicInstanceMethod<StringBuilder, string>(\"Append\", true).IfNone(() => throw new ArgumentException($\"Append method found for StringBuilder\"));\n        var toString      = GetPublicInstanceMethod<StringBuilder>(\"ToString\", false).IfNone(() => throw new ArgumentException($\"ToString method found for StringBuilder\"));\n        var name          = typeof(A).Name;\n        var self          = Expression.Parameter(typeof(A), \"self\");\n        var nullA         = Expression.Constant(null, typeof(A));\n        var nullStr       = Expression.Constant(null, typeof(string));\n        var sb            = Expression.Variable(typeof(StringBuilder), \"sb\");\n        var tmpStr        = Expression.Variable(typeof(string), \"tmpStr\");\n        var result        = Expression.Variable(typeof(string), \"result\");\n        var returnTarget  = Expression.Label(typeof(string));\n            \n        if (name.IndexOf('`') != -1) name = name.Split('`').AsIterable().Head.Value!;\n\n        Expression fieldExpr(FieldInfo field)\n        {\n            var convertToString = (GetPublicStaticMethod(typeof(Convert), \"ToString\", field.FieldType) ||\n                                   GetPublicStaticMethod(typeof(Convert), \"ToString\", typeof(object)))\n               .IfNone(() => throw new Exception());","sourceCodeStart":762,"sourceCodeEnd":798,"githubUrl":"https://github.com/louthy/language-ext/blob/2f0e3628242889774d4141960a35671a0280051f/LanguageExt.Core/Utility/IL.cs#L762-L798","documentation":"Sentinel guard in ToStringExpr<A>: the reflection lookup for StringBuilder.Append(char)/Append(string) returned None while compiling the ToString lambda for type A. The message is inverted ('found' means 'not found'); it signals an environment invariant failure, never a user-triggerable condition.","triggerScenarios":"Thrown at LanguageExt.Core/Utility/IL.cs:780 when the library encounters an invalid state.","commonSituations":"See trigger scenarios.","solutions":["Catch the failure at Show-derivation time and fall back to a simple formatter that does not rely on expression compilation over StringBuilder.","Check runtime trimming/AOT configuration: ensure StringBuilder's Append overloads are preserved so reflection-based expression building can find them.","Pin or align the LanguageExt version with the runtime, since reflection member resolution can differ across .NET versions and break the generated expression."],"exampleFix":null,"handlingStrategy":"fallback","validationCode":null,"typeGuard":null,"tryCatchPattern":null,"preventionTips":[],"tags":[],"backgroundTag":null,"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"}