How do I pass a parameter by reference to managed code in C#?

Question

I'm using a DLL with the following function in C# Visual Studio 2008:

[DllImport("slpapi62", EntryPoint = "_SlpGetPrinterName@12")]
public static extern int SlpGetPrinterName(int nIndex, string strPrinterName, int nMaxChars);

Calling this function, strPrinterName is suppose to return a string.

string name = "";
SlpGetPrinterName(0, name, 128);

How can i get this parameter to pass by reference?

Answer 1

Pass a StringBuilder object instead of a string:

[DllImport("slpapi62", EntryPoint = "_SlpGetPrinterName@12")]
public static extern int SlpGetPrinterName(int nIndex, StringBuilder strPrinterName, int nMaxChars);

Calling it:

StringBuilder name = new StringBuilder(128);
int value = SlpGetPrinterName(0, name, name.Capacity);

Answer 2

使用StringBuilder对象作为string参数。

Answer 3

Can you use the ref keyword?

string name = "";
SlpGetPrinterName(0, ref name, 128);

There is a detail explanation of passing variables by reference here http://www.yoda.arachsys.com/csharp/parameters.html

Answer 4

It sounds like you need the name value to be set by the external code. It has been some time since I have done any pInvoke, but I believe the following is the correct signature:

[DllImport("slpapi62", EntryPoint = "_SlpGetPrinterName@12")]
public static extern int SlpGetPrinterName(int nIndex, out string strPrinterName, int nMaxChars);

Note the 'out' keyword before 'string strPrinterName'. You would then call it like so:

string name;
SlpGetPrinterName(0, out name, 128);