Why do invalid cast exceptions occur when executing MATLAB R2022b functions from Visual Basic .NET applications?

I am using the .NET class "MATLABEngine" to call MATLAB functions as methods from a Visual Basic .NET console app, as described in the documentation below:

https://www.mathworks.com/help/releases/R2022b/matlab/apiref/mathworks.matlab.engine.matlabengine.html

I'm able to get the expected output shown in the Visual Basic .NET example for the "StartMATLAB" method, which starts a new MATLAB process with default options.

However, when I add the following lines of code that call the "magic" MATLAB function to the Visual Basic .NET example code:

Dim xIn As Double = 5.0
Dim results(,) As Double
results = matlab.magic(xIn)

I receive an invalid cast exception as shown below:

Unhandled exception. System.InvalidCastException: Unable to cast object of type 'MathWorks.MATLAB.Types.FevalTaskResult' to type 'System.Double[,]'.
   at <NAME OF CONSOLE APP>.Program.Main() in <PATH TO PROGRAM FILE CONTAINING EXAMPLE CODE>:<LINE 3 FROM EXAMPLE CODE>

I would like to resolve this error to get a better understanding of building Visual Basic .NET console apps that leverage MATLAB functions.

Answer 1

[MathWorks Support Team]

In Visual Basic .NET, you have to use the "CTypeDynamic" method to convert variables of dynamic type (e.g. the return of "matlab.magic()") to a known type (e.g. Double(,)). For more information regarding the "CTypeDynamic" method, take a look at Microsoft's documentation linked below:

https://learn.microsoft.com/en-us/dotnet/api/microsoft.visualbasic.conversion.ctypedynamic?view=net-7.0#microsoft-visualbasic-conversion-ctypedynamic(system-object-system-type)

For example, modify the line:

results = matlab.magic(xIn)

To:

results = CTypeDynamic(matlab.magic(xIn), GetType(Double(,)))

As of R2022b, this is necessary for writing Visual Basic .NET code due to dynamic typing, and unfortunately there is no way to make the code less verbose at this time. A code example is attached with the file "Program.vb".