I have ported some old BC++ code to Managed C++/CLI code and build it to a dll

Now I trie to call a method from this dll in my C# code. But this does not work. The compiler gives the following error:
'TProjLicense' does not contain a definition for 'CalcSignature' and no extension method 'CalcSignature' accepting a first argument of type 'TProjLicense' could be found (are you missing a using directive or an assembly reference?)

The code (C++/CLI) in my DLL is as follows:
public class TProjLicense
// Constructor
TProjLicense() {};

String ^ TProjLicense::CalcSignature(String ^ CustName, array<String ^> ^Proj,
int NumProj);


And from C# I am calling it as follows:

// Make a list of all project names
String [] ProjectList = new String [10];
//ProjectList[1] = cmbProject1->Items->Strings[cmbProject1.SelectedIndex];
ProjectList[0] = cmbProject1.SelectedText;
ProjectList[1] = cmbProject2.SelectedText;
// Generate the signature;
TProjLicense License;
edtSignature.Text = License.CalcSignature(edtCustomerName.Text,
ProjectList, 10);

Probably there is something with the string array, but I am new in CLI/C# so I don't have a clue how to solve this.
Has anyone an idea ?

Thanks already.