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AppDomain.AssemblyResolve Event

Occurs when the resolution of an assembly fails.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)

public event ResolveEventHandler AssemblyResolve

Implements

_AppDomain.AssemblyResolve

It is the responsibility of the ResolveEventHandler for this event to return the assembly that is specified by the ResolveEventArgs.Name property, or to return null if the assembly is not recognized. The assembly must be loaded into an execution context; if it is loaded into the reflection-only context, the load that caused this event to be raised fails.

For guidance on the use of this event, see Resolving Assembly Loads.

Beginning with the .NET Framework 4, the ResolveEventArgs.RequestingAssembly property returns the assembly that requested the assembly load that could not be resolved. For example, the loader might be unable to load a dependency of the requesting assembly because the requesting assembly and its dependency are not in the probing path. Knowing the identity of the requesting assembly might be useful in locating the dependency or in identifying the correct version, if more than one version of the dependency is available. For more information, see ResolveEventArgs.RequestingAssembly.

Important noteImportant

Beginning with the .NET Framework 4, the ResolveEventHandler event is raised for all assemblies, including resource assemblies. In earlier versions, the event was not raised for resource assemblies. If the operating system is localized, the handler might be called multiple times: once for each culture in the fallback chain.

For this event, the ResolveEventArgs.Name property returns the assembly name before policy is applied.

Important noteImportant

If more than one event handler is registered for this event, the event handlers are called in order until an event handler returns a value that isn't null. Subsequent event handlers are ignored.

For more information about handling events, see Consuming Events.

The following sample demonstrates the AssemblyResolve event.

For this code example to run, you must provide the fully qualified assembly name. For information about how to obtain the fully qualified assembly name, see Assembly Names.

public class MyType
{
    public MyType()
    {
        Console.WriteLine();
        Console.WriteLine("MyType instantiated!");
    }
}

class Test
{
    public static void Main()
    {
        AppDomain currentDomain = AppDomain.CurrentDomain;

        // This call will fail to create an instance of MyType since the 
        // assembly resolver is not set
        InstantiateMyTypeFail(currentDomain);

        currentDomain.AssemblyResolve += new ResolveEventHandler(MyResolveEventHandler);

        // This call will succeed in creating an instance of MyType since the 
        // assembly resolver is now set.
        InstantiateMyTypeFail(currentDomain);

        // This call will succeed in creating an instance of MyType since the 
        // assembly name is valid.
        InstantiateMyTypeSucceed(currentDomain);
    }

    private static void InstantiateMyTypeFail(AppDomain domain)
    {
        // Calling InstantiateMyType will always fail since the assembly info 
        // given to CreateInstance is invalid. 
        try
        {
            // You must supply a valid fully qualified assembly name here.
            domain.CreateInstance("Assembly text name, Version, Culture, PublicKeyToken", "MyType");
        }
        catch (Exception e)
        {
            Console.WriteLine();
            Console.WriteLine(e.Message);
        }
    }

    private static void InstantiateMyTypeSucceed(AppDomain domain)
    {
        try
        {
            string asmname = Assembly.GetCallingAssembly().FullName;
            domain.CreateInstance(asmname, "MyType");
        }
        catch (Exception e)
        {
            Console.WriteLine();
            Console.WriteLine(e.Message);
        }
    }

    private static Assembly MyResolveEventHandler(object sender, ResolveEventArgs args)
    {
        Console.WriteLine("Resolving...");
        return typeof(MyType).Assembly;
    }
}

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

  • SecurityCriticalAttribute 

    Requires full trust for the immediate caller. This member cannot be used by partially trusted or transparent code.

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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