使用user32.dll从外部应用程序的文本框（Unicode）中将文本提取到C＃应用程序中

I have developed an Application in C# which extract text from external application's textbox , I am using user32.dll, The application is working fine but my problem is this - The external application's textbox contains text in unicode format, so whenever I extract text in my application it shows "??????" text. I have tried setting charset.unicode , and also used RichTextBox to show text in my application. Please let me know how to exract unicode text from external application. I have developed an Application in C# which ext