How to add folder to assembly search path at runtime in .NET?

Question

Sounds like you could use the AppDomain.AssemblyResolve event and manually load the dependencies from your DLL directory.

Edit (from the comment):

AppDomain currentDomain = AppDomain.CurrentDomain;
currentDomain.AssemblyResolve += new ResolveEventHandler(LoadFromSameFolder);

static Assembly LoadFromSameFolder(object sender, ResolveEventArgs args)
{
    string folderPath = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
    string assemblyPath = Path.Combine(folderPath, new AssemblyName(args.Name).Name + ".dll");
    if (!File.Exists(assemblyPath)) return null;
    Assembly assembly = Assembly.LoadFrom(assemblyPath);
    return assembly;
}

You can add a probing path to your application's .config file, but it will only work if the probing path is a contained within your application's base directory.

Update for Framework 4

Since Framework 4 raise the AssemblyResolve event also for resources actually this handler works better. It's based on the concept that localizations are in app subdirectories (one for localization with the name of the culture i.e. C:\MyApp\it for Italian) Inside there are resources file. The handler works also if the localization is country-region i.e. it-IT or pt-BR. In this case the handler "might be called multiple times: once for each culture in the fallback chain" [from MSDN]. This means that if we return null for "it-IT" resource file the framework raises the event asking for "it".

Event hook

        AppDomain currentDomain = AppDomain.CurrentDomain;
        currentDomain.AssemblyResolve += new ResolveEventHandler(currentDomain_AssemblyResolve);

Event handler

    Assembly currentDomain_AssemblyResolve(object sender, ResolveEventArgs args)
    {
        //This handler is called only when the common language runtime tries to bind to the assembly and fails.

        Assembly executingAssembly = Assembly.GetExecutingAssembly();

        string applicationDirectory = Path.GetDirectoryName(executingAssembly.Location);

        string[] fields = args.Name.Split(',');
        string assemblyName = fields[0];
        string assemblyCulture;
        if (fields.Length < 2)
            assemblyCulture = null;
        else
            assemblyCulture = fields[2].Substring(fields[2].IndexOf('=') + 1);


        string assemblyFileName = assemblyName + ".dll";
        string assemblyPath;

        if (assemblyName.EndsWith(".resources"))
        {
            // Specific resources are located in app subdirectories
            string resourceDirectory = Path.Combine(applicationDirectory, assemblyCulture);

            assemblyPath = Path.Combine(resourceDirectory, assemblyFileName);
        }
        else
        {
            assemblyPath = Path.Combine(applicationDirectory, assemblyFileName);
        }



        if (File.Exists(assemblyPath))
        {
            //Load the assembly from the specified path.                    
            Assembly loadingAssembly = Assembly.LoadFrom(assemblyPath);

            //Return the loaded assembly.
            return loadingAssembly;
        }
        else
        {
            return null;
        }

    }

The best explanation from MS itself:

AppDomain currentDomain = AppDomain.CurrentDomain;
currentDomain.AssemblyResolve += new ResolveEventHandler(MyResolveEventHandler);

private Assembly MyResolveEventHandler(object sender, ResolveEventArgs args)
{
    //This handler is called only when the common language runtime tries to bind to the assembly and fails.

    //Retrieve the list of referenced assemblies in an array of AssemblyName.
    Assembly MyAssembly, objExecutingAssembly;
    string strTempAssmbPath = "";

    objExecutingAssembly = Assembly.GetExecutingAssembly();
    AssemblyName[] arrReferencedAssmbNames = objExecutingAssembly.GetReferencedAssemblies();

    //Loop through the array of referenced assembly names.
    foreach(AssemblyName strAssmbName in arrReferencedAssmbNames)
    {
        //Check for the assembly names that have raised the "AssemblyResolve" event.
        if(strAssmbName.FullName.Substring(0, strAssmbName.FullName.IndexOf(",")) == args.Name.Substring(0, args.Name.IndexOf(",")))
        {
            //Build the path of the assembly from where it has to be loaded.                
            strTempAssmbPath = "C:\\Myassemblies\\" + args.Name.Substring(0,args.Name.IndexOf(","))+".dll";
            break;
        }

    }

    //Load the assembly from the specified path.                    
    MyAssembly = Assembly.LoadFrom(strTempAssmbPath);                   

    //Return the loaded assembly.
    return MyAssembly;          
}

For C++/CLI users, here is @Mattias S' answer (which works for me):

using namespace System;
using namespace System::IO;
using namespace System::Reflection;

static Assembly ^LoadFromSameFolder(Object ^sender, ResolveEventArgs ^args)
{
    String ^folderPath = Path::GetDirectoryName(Assembly::GetExecutingAssembly()->Location);
    String ^assemblyPath = Path::Combine(folderPath, (gcnew AssemblyName(args->Name))->Name + ".dll");
    if (File::Exists(assemblyPath) == false) return nullptr;
    Assembly ^assembly = Assembly::LoadFrom(assemblyPath);
    return assembly;
}

// put this somewhere you know it will run (early, when the DLL gets loaded)
System::AppDomain ^currentDomain = AppDomain::CurrentDomain;
currentDomain->AssemblyResolve += gcnew ResolveEventHandler(LoadFromSameFolder);

I've used @Mattias S' solution. If you actually want to resolve dependencies from the same folder - you should try using Requesting assembly location, as shown below. args.RequestingAssembly should be checked for nullity.

System.AppDomain.CurrentDomain.AssemblyResolve += (s, args) =>
{
    var loadedAssembly = System.AppDomain.CurrentDomain.GetAssemblies().Where(a => a.FullName == args.Name).FirstOrDefault();
    if(loadedAssembly != null)
    {
        return loadedAssembly;
    }

    if (args.RequestingAssembly == null) return null;

    string folderPath = Path.GetDirectoryName(args.RequestingAssembly.Location);
    string rawAssemblyPath = Path.Combine(folderPath, new System.Reflection.AssemblyName(args.Name).Name);

    string assemblyPath = rawAssemblyPath + ".dll";

    if (!File.Exists(assemblyPath))
    {
        assemblyPath = rawAssemblyPath + ".exe";
        if (!File.Exists(assemblyPath)) return null;
    } 

    var assembly = System.Reflection.Assembly.LoadFrom(assemblyPath);
    return assembly;
 };