SCardGetCardTypeProviderName return empty results

Question

I'm trying to use the SCardGetCardTypeProviderName using interop in C#. One of the parameters is a reference, that is supposed to return the name of the provider for a smart card according to the card context that is passed in. This is the code I'm using:

IntPtr hSC = { value comes from call using SCardEstablishContext }
string cardName = { value comes from SCardUIDlgSelectCard }
int providerNameLength = 256;
string providerName = string.Empty;  //doesn't matter how I initialize this

[DllImport("winscard.dll", CharSet = CharSet.Ansi, SetLastError = true)]
public static extern Int32 SCardGetCardTypeProviderName(IntPtr hContext, string szCardName, uint dwProviderId, ref string szProvider, ref int pcchProvider);

int lReturn = SCardGetCardTypeProviderName(hSC, cardName, SCARD_PROVIDER_CSP, ref providerName, ref providerNameLength);

But the providerName always comes back empty, while the providerNameLength changes to 43, which makes me think it's a marshalling problem. But I've tried marshalling providerName as:

• LPWStr
• LPTStr
• LPStr

In the case of the latter two, instead of an empty string, I get a string of strange characters, again, making me think it's a marshalling/translation issue.

Answer 1

Declare the string parameter as StringBuilder:

[DllImport("winscard.dll"]
public static extern int SCardGetCardTypeProviderName(
    IntPtr hContext,
    string szCardName, 
    uint dwProviderId, 
    StringBuilder szProvider, 
    ref int pcchProvider
);
....
StringBuilder providerName = new StringBuilder(providerNameLength);
int lReturn = SCardGetCardTypeProviderName(
    hSC, 
    cardName, 
    SCARD_PROVIDER_CSP, 
    providerName, 
    ref providerNameLength
);

No need to repeat the default value of CharSet, and these API functions don't use the Win32 last error mechanism.