PSObject.ToString mishandles TypeCode, throws InvalidCastException for otherwise good objects #28034

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1. Ji Luo (GeeLaw)

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   Needs-TriageThe issue is new and needs to be triaged by a work group.The issue is new and needs to be triaged by a work group.

   on Sep 19, 2026
2. GeeLaw commented on Sep 19, 2026

   

   Ji Luo (GeeLaw)

   on Sep 19, 2026

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   In PSObject.ToString, PowerShell tries to fast-track “common simple” types. It detects whether value is a common simple type using Convert.GetTypeCode(value). If it returns TypeCode.String, the code then tries to cast value to string (and to IFormattable or concrete types for other various type codes).

   This is wrong. There are primarly 3 methods to get TypeCode, with semantics explained below.

   • TypeCode IConvertible.GetTypeCode() is an interface method hence polymorphic on each object. If an object implements IConvertible, its GetTypeCode is supposed to return the type code describing the data it wraps (if it wraps a single value of a common simple type), or TypeCode.Object if not applicable. Notably, if T implements IConvertible, there’s no reason to believe all instances of T return the same TypeCode from this method.
   • TypeCode Type.GetTypeCode(Type type) is a static method and obtains the type code of type statically, which is determined as follows: if it’s the actual “common simple type” corresponding to a TypeCode (e.g., int/string for Int32/String), then return it; if it’s an enum, then return the underlying type’s type code; otherwise, return TypeCode.Object. This method doesn’t care if Type implements IConvertible.
   • TypeCode Convert.GetTypeCode(object obj) is a static method and serves as a universal way to determine the type code polymorphically, falling back to TypeCode.Object if obj doesn’t implement IConvertible.

   It is perfectly valid for a type that is not string to implement IConvertible and return TypeCode.String, e.g., a UTF-8/32 string type. It’s also perfectly valid if a type returns TypeCode.UInt32 without being IFormattable, or returns TypeCode.Double without being double.

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   The correct type code to check is Type.GetTypeCode(obj.GetType()) (PowerShell has an internal extension method on Type so that you can say obj.GetType().GetTypeCode()). Note that Windows PowerShell 5.1 doesn’t have this bug, because it does the right thing.

   This bug was introduced 7 years ago in PowerShell/PowerShell#9893, affecting PowerShell 7.0.0-preview.2 to 7.6.6 and 7.7.0-preview.1 to 7.7.0-preview.4.
3. GeeLaw commented on Sep 19, 2026

   

   Ji Luo (GeeLaw)

   on Sep 19, 2026

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   I am happy to offer a PR for this issue. If you’re happy to let me take a stab, please assign this issue to me.

   Cross-referencing: dotnet/runtime#134285