See the question and my original answer on StackOverflow

Here is a piece of code that allows to do this, and an exemple:

    string path = GetAssemblyPath("System.DirectoryServices");

Note if you need a specific processor architecture, since it supports partial name, you can write this kind of things:

    // load from the 32-bit GAC
    string path = GetAssemblyPath("Microsoft.Transactions.Bridge.Dtc, ProcessorArchitecture=X86");

    // load from the 64-bit GAC
    string path = GetAssemblyPath("Microsoft.Transactions.Bridge.Dtc, ProcessorArchitecture=AMD64");

This is the implementation:

    /// <summary>
    /// Gets an assembly path from the GAC given a partial name.
    /// </summary>
    /// <param name="name">An assembly partial name. May not be null.</param>
    /// <returns>
    /// The assembly path if found; otherwise null;
    /// </returns>
    public static string GetAssemblyPath(string name)
        if (name == null)
            throw new ArgumentNullException("name");

        string finalName = name;
        AssemblyInfo aInfo = new AssemblyInfo();
        aInfo.cchBuf = 1024; // should be fine...
        aInfo.currentAssemblyPath = new String('\0', aInfo.cchBuf);

        IAssemblyCache ac;
        int hr = CreateAssemblyCache(out ac, 0);
        if (hr >= 0)
            hr = ac.QueryAssemblyInfo(0, finalName, ref aInfo);
            if (hr < 0)
                return null;

        return aInfo.currentAssemblyPath;

    [ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("e707dcde-d1cd-11d2-bab9-00c04f8eceae")]
    private interface IAssemblyCache
        void Reserved0();

        int QueryAssemblyInfo(int flags, [MarshalAs(UnmanagedType.LPWStr)] string assemblyName, ref AssemblyInfo assemblyInfo);

    private struct AssemblyInfo
        public int cbAssemblyInfo;
        public int assemblyFlags;
        public long assemblySizeInKB;
        public string currentAssemblyPath;
        public int cchBuf; // size of path buf.

    private static extern int CreateAssemblyCache(out IAssemblyCache ppAsmCache, int reserved);