Diagnosing Windows Runtime component error conditions

This article provides additional information about restrictions on Windows Runtime Components written with managed code. It expands on the information that is provided in error messages from Winmdexp.exe (Windows Runtime Metadata Export Tool), and complements the information on restrictions that is provided in Creating Windows Runtime Components in C# and Visual Basic.

This article doesn’t cover all errors. The errors discussed here are grouped by general category, and each category includes a table of associated error messages. Search for message text (omitting specific values for placeholders) or for message number. If you don’t find the information you need here, please help us improve the documentation by using the feedback button at the end of this article. Include the error message. Alternatively, you can file a bug at the Microsoft Connect website.

This article contains the following sections:

• When implementing async pattern, error message provides incorrect type

• Missing references to mscorlib.dll and/or System.Runtime.dll

• Operator overloading is not allowed

• Constructors on a class have the same number of parameters

• Must specify a default for overloads that have the same number of parameters

• Namespace errors and invalid names for the output file

• Exporting types that aren't valid Windows Runtime types

• Structures that contain fields of disallowed types

• Restrictions on arrays in member signatures

• Array parameters must specify whether array contents are readable or writable

• Member with a parameter named "value"

Error message for implementing async interface provides incorrect type

Managed Windows Runtime components cannot implement the Windows Runtime interfaces that represent asynchronous actions or operations (IAsyncAction, IAsyncActionWithProgress<TProgress>, IAsyncOperation<TResult>, or IAsyncOperationWithProgress<TResult, TProgress>). Instead, the .NET Framework provides the AsyncInfo class for generating async operations in Windows Runtime components. The error message that Winmdexp.exe displays when you try to implement an async interface incorrectly refers to this class by its former name, AsyncInfoFactory. The .NET Framework no longer includes the AsyncInfoFactory class.

Missing references to mscorlib.dll or System.Runtime.dll

This issue occurs only when you use Winmdexp.exe from the command line. We recommend that you use the /reference option to include references to both mscorlib.dll and System.Runtime.dll from the .NET Framework core reference assemblies, which are located in "%ProgramFiles(x86)%\Reference Assemblies\Microsoft\Framework\.NETCore\v4.5" ("%ProgramFiles%\..." on a 32-bit computer).

Operator overloading is not allowed

In a Windows Runtime component written in managed code, you cannot expose overloaded operators on public types.

Constructors on a class have the same number of parameters

In the Windows Runtime, a class can have only one constructor with a given number of parameters; for example, you can't have one constructor that has a single parameter of type String and another that has a single parameter of type int (Integer in Visual Basic). The only workaround is to use a different number of parameters for each constructor.

Must specify a default for overloads that have the same number of parameters

In the Windows Runtime, overloaded methods can have the same number of parameters only if one overload is specified as the default overload. See "Overloaded Methods" in Creating Windows Runtime Components in C# and Visual Basic.

Namespace errors and invalid names for the output file

In the Windows Runtime, all the public types in a Windows metadata (.winmd) file must be in a namespace that shares the .winmd file name, or in sub-namespaces of the file name. For example, if your Visual Studio 2012 project is named A.B (that is, your Windows Runtime component is A.B.winmd), it can contain public classes A.B.Class1 and A.B.C.Class2, but not A.Class3 (WME0006) or D.Class4 (WME1044).

In the case of A.Class3, you can either move Class3 to another namespace or change the name of the Windows Runtime component to A.winmd. Although WME0006 is a warning, you should treat it as an error. In the previous example, code that calls A.B.winmd will be unable to locate A.Class3. 

In the case of D.Class4, no file name can contain both D.Class4 and classes in the A.B namespace, so changing the name of the Windows Runtime component is not an option. You can either move D.Class4 to another namespace, or put it in another Windows Runtime component.

The file system can't distinguish between uppercase and lowercase, so namespaces that differ by case are not allowed (WME1067).

Your component must contain at least one public sealed type (Public NotInheritable in Visual Basic). If not, you will get WME1042 or WME1043, depending on whether your component contains private types.

A type in a Windows Runtime component cannot have a name that is the same as a namespace (WME1068).

Exporting types that aren't valid Windows Runtime types

The public interface of your component must expose only Windows Runtime types. However, the .NET Framework provides mappings for a number of commonly used types that are slightly different in the .NET Framework and the Windows Runtime. This enables the .NET Framework developer to work with familiar types instead of learning new ones. You can use these mapped .NET Framework types in the public interface of your component. See "Declaring types in Windows Runtime Components" and "Passing Windows Runtime types to managed code" in Creating Windows Runtime Components in C# and Visual Basic, and .NET Framework mappings of Windows Runtime types.

Many of these mappings are interfaces. For example, IList<T><T><'T>(Of T) maps to the Windows Runtime interface IVector<T>. If you use List<string> (List(Of String) in Visual Basic) instead of IList<string> as a parameter type, Winmdexp.exe provides a list of alternatives that includes all the mapped interfaces implemented by List<T><T><'T>(Of T). If you use nested generic types, such as List<Dictionary<int, string>> (List(Of Dictionary(Of Integer, String)) in Visual Basic), Winmdexp.exe offers choices for each level of nesting. These lists can become quite long.

In general, the best choice is the interface that is closest to the type. For example, for Dictionary<int, string>, the best choice is most likely IDictionary<int, string>.

Error number	Message text
WME1033	Method '{0}' has parameter '{1}' of type '{2}'. '{2}' is not a valid Windows Runtime parameter type.
WME1038	Method '{0}' has a parameter of type '{1}' in its signature. Although this type is not a valid Windows Runtime type, it implements interfaces that are valid Windows Runtime types. Consider changing the method signature to use one of the following types instead: '{2}'.
WME1039	Method '{0}' has a parameter of type '{1}' in its signature. Although this generic type is not a valid Windows Runtime type, the type or its generic parameters implement interfaces that are valid Windows Runtime types. {2} Note For {2}, Winmdexp.exe appends a list of alternatives, such as "Consider changing the type 'System.Collections.Generic.List<T>' in the method signature to one of the following types instead: 'System.Collections.Generic.IList<T>, System.Collections.Generic.IReadOnlyList<T>, System.Collections.Generic.IEnumerable<T>'."
WME1040	Method '{0}' has a parameter of type '{1}' in its signature. Instead of using a managed Task type, use Windows.Foundation.IAsyncAction, Windows.Foundation.IAsyncOperation, or one of the other Windows Runtime async interfaces. The standard .NET await pattern also applies to these interfaces. Please see System.Runtime.InteropServices.WindowsRuntime.AsyncInfo for more information about converting managed task objects to Windows Runtime async interfaces.

Structures that contain fields of disallowed types

In the Windows Runtime, a structure can contain only fields, and only structures can contain fields. Those fields must be public. Valid field types include enumerations, structures, and primitive types.

Restrictions on arrays in member signatures

In the Windows Runtime, arrays in member signatures must be one-dimensional with a lower bound of 0 (zero). Nested array types such as myArray[][] (myArray()() in Visual Basic) are not allowed.

Array parameters must specify whether array contents are readable or writable

In the Windows Runtime, parameters must be read-only or write-only. Parameters cannot be marked ref (ByRef without the OutAttribute attribute in Visual Basic). This applies to the contents of arrays, so array parameters must indicate whether the array contents are read-only or write-only. The direction is clear for out parameters (ByRef parameter with the OutAttribute attribute in Visual Basic), but array parameters that are passed by value (ByVal in Visual Basic) must be marked. See Passing arrays to a Windows Runtime component.

Member with a parameter named "value"

In the Windows Runtime, return values are considered to be output parameters, and the names of parameters must be unique. By default, Winmdexp.exe gives the return value the name "value". If your method has a parameter named "value", you will get error WME1092. There are two ways to correct this:

• Give your parameter a name other than "value" (in property accessors, a name other than "returnValue").

• Use the ReturnValueNameAttribute attribute to change the name of the return value, as shown here:

  using System.Runtime.InteropServices;
  using System.Runtime.InteropServices.WindowsRuntime;
  
  [return: ReturnValueName("average")]
  public int GetAverage(out int lowValue, out int highValue)
  
  Imports System.Runtime.InteropServices
  Imports System.Runtime.InteropServices.WindowsRuntime
  
  Public Function GetAverage(<Out> ByRef lowValue As Integer, _
  <Out> ByRef highValue As Integer) As <ReturnValueName("average")> String
  

JavaScript code can access the output parameters of a method by name, including the return value. For an example, see the ReturnValueNameAttribute attribute.

Error number	Message text
WME1091	The method '{0}' has the return value named '{1}' which is the same as a parameter name. Windows Runtime method parameters and return value must have unique names.
WME1092	The method '{0}' has a parameter named '{1}' which is the same as the default return value name. Consider using another name for the parameter or use the System.Runtime.InteropServices.WindowsRuntime.ReturnValueNameAttribute to explicitly specify the name of the return value. Note The default name is "returnValue" for property accessors and "value" for all other methods.

See Also

Creating Windows Runtime Components in C# and Visual Basic
Winmdexp.exe (Windows Runtime Metadata Export Tool)