CA2101: Specify marshaling for P/Invoke string arguments

The new home for Visual Studio documentation is Visual Studio 2017 Documentation on docs.microsoft.com.

The latest version of this topic can be found at CA2101: Specify marshaling for P-Invoke string arguments.

TypeName|SpecifyMarshalingForPInvokeStringArguments|
|CheckId|CA2101|
|Category|Microsoft.Globalization|
|Breaking Change|Non-breaking|

A platform invoke member allows for partially trusted callers, has a string parameter, and does not explicitly marshal the string.

When you convert from Unicode to ANSI, it is possible that not all Unicode characters can be represented in a specific ANSI code page. Best-fit mapping tries to solve this problem by substituting a character for the character that cannot be represented. The use of this feature can cause a potential security vulnerability because you cannot control the character that is chosen. For example, malicious code could intentionally create a Unicode string that contains characters that are not found in a particular code page, which are converted to file system special characters such as '..' or '/'. Note also that security checks for special characters frequently occur before the string is converted to ANSI.

Best-fit mapping is the default for the unmanaged conversion, WChar to MByte. Unless you explicitly disable best-fit mapping, your code might contain an exploitable security vulnerability because of this issue.

To fix a violation of this rule, explicitly marshal string data types.

Do not suppress a warning from this rule.

The following example shows a method that violates this rule, and then shows how to fix the violation.

using System;
using System.Runtime.InteropServices;
[assembly: System.Security.AllowPartiallyTrustedCallers()]

namespace SecurityLibrary
{
    class NativeMethods
    {
        // Violates rule: SpecifyMarshalingForPInvokeStringArguments.
        [DllImport("advapi32.dll", CharSet=CharSet.Auto)]
        internal static extern int RegCreateKey(IntPtr key, String subKey, out IntPtr result);

        // Satisfies rule: SpecifyMarshalingForPInvokeStringArguments.
        [DllImport("advapi32.dll", CharSet = CharSet.Unicode)]
        internal static extern int RegCreateKey2(IntPtr key, String subKey, out IntPtr result);
    }
}