[C]Dll in csharp

Hi,

I have a dll coding in C langage and I want to use it in Csharp. This
is my declaration in csharp :
[DllImport("myDll.dll")]
public static extern unsafe string
myFunction([MarshalAs(UnmanagedType.LPArray)] byte[] myData, int width,
int height);

I call in csharp like this :
try{
string test = myFunction(tempArray, 320, 240); and
tempArray is correctly filled.
}
catch {
Console.Writeline("Error in fonction DLL");
}

My C function seems like that:
char *myFunction(char *myData, int Arg_width, int Arg_height)
{
char *myResult;

//malloc of myResult
myResult = malloc(strlen(myData) * sizeof (char));

printf("I fill myResult.");

*(myData) = '\0';

printf("I return my data");
return (myData);
}



And it print :
I fill myResult.
I return my data.
Error in fonction DLL

Can anyome explain why Csharp get an error ?