Assembly.Load,LoadFile,LoadFrom

在加载Assembly的时候有三种Load Context可以选择：
Load Context
通常来说，当通过探测 GAC，host assembly store (if hosted), 或者 the ApplicationBase / PrivateBinPaths of the AppDomain来找到指定的Assembly时，被加载到Load Context中。
LoadFrom Context
通常来说，如果用户提供一个path来找到某个Assembly（并且在那个path中的Assembly没有在Load Context中找到），那么这个Assembly就被加载到LoadFrom Context中。有多个方法可以使用Path加载Assembly：LoadFrom(), CreateInstanceFrom(), ExecuteAssembly(), loading an assembly through interop using a codebase, etc.
Neither Context
If the user generated or found the assembly instead of Fusion, it's in neither context. This applies to assemblies loaded by Assembly.Load(byte[]) and Reflection Emit assemblies (that haven't been loaded from disk). Assembly.LoadFile() assemblies are also generally loaded into this context, even though a path is given (because it doesn't go through Fusion).
注意： Assembly的三个方法并不与这三个Context严格对应，比如Assembly.Load(byte[]) 并不是用Load Context加载的。
三种Context的比较可以参考下面的表格：

	Advantages	Disadvantages
Load	Gets the benefits of versioning and policy. Best avoids "Dll Hell." Dll Hell can break your app over time. Dependencies available in the Load context are automatically found.	Dependencies in other contexts are not available unless you subscribe to the AppDomain.AssemblyResolve event.
LoadFrom	Assemblies can be loaded from multiple paths, not just from beneath the ApplicationBase. Dependencies already loaded in this context will automatically be found. Dependencies in the same dir as the requesting LoadFrom context assembly will automatically be found.	If a Load context assembly tries to load this assembly by display name, it will fail to be found by default (e.g., when mscorlib.dll deserializes this assembly). Worse, an assembly with the same identity but at a different path could be found on the probing path, causing an InvalidCastException, MissingMethodException, or unexpected method behavior later on. If an assembly by the same identity is already loaded, you'll get that one back, even if you've specified a path to a different one. It does a FileIOPermission.Read + PathDiscovery demand, or a WebPermission demand on the path/URL specified. The ngen image (if any) won't be used. Can't be loaded as domain-neutral. v1.0 and v1.1 CLR only: won't be affected by policy, so can't be centrally serviced.
Neither	Avoids probing costs for loading this assembly. You can load multiple assemblies with the same identity into the same appdomain. You get the bits from the location you specified (as opposed to LoadFrom). Starting in v2, policy/GAC overrides it, however.	Nothing can bind to this assembly unless you've subscribed to the AssemblyResolve event. Dependencies can only be loaded from the Load context or using the AssemblyResolve event. Passing this assembly to another AppDomain may become tricky (e.g., when this is a Reflection Emit assembly that hasn't been saved). The ngen image (if any) won't be used. Can't be loaded as domain-neutral. v1.0 and v1.1 CLR only: won't be affected by policy, so can't be centrally serviced.

public static Assembly Load(
    string assemblyString
)
public static Assembly Load(
    byte[] rawAssembly  //Assembly的二进制形式
)
public static Assembly Load(
    AssemblyName assemblyRef
)
public static Assembly Load(
    byte[] rawAssembly, //Assembly的二进制形式
    byte[] rawSymbolStore //Assembly Symbol的二进制形式
)
public static Assembly Load(
    AssemblyName assemblyRef,
    Evidence assemblySecurity
)
public static Assembly Load(
    string assemblyString,
    Evidence assemblySecurity
)
[SecurityPermissionAttribute(SecurityAction.Demand, Flags = SecurityPermissionFlag.ControlEvidence)]
public static Assembly Load(
    byte[] rawAssembly,
    byte[] rawSymbolStore,
    Evidence securityEvidence
)

public static Assembly LoadFrom(
    string assemblyFile
)
public static Assembly LoadFrom(
    string assemblyFile,
    Evidence securityEvidence
)
public static Assembly LoadFrom(
    string assemblyFile,
    Evidence securityEvidence,
    byte[] hashValue,
    AssemblyHashAlgorithm hashAlgorithm
)

public static Assembly LoadFile(
    string path
)

[SecurityPermissionAttribute(SecurityAction.Demand, Flags = SecurityPermissionFlag.ControlEvidence)]
public static Assembly LoadFile(
    string path,
    Evidence securityEvidence
)

Ref:
http://blogs.msdn.com/suzcook/archive/2003/05/29/57143.aspx
http://blogs.msdn.com/suzcook/archive/2003/09/19/57248.aspx
http://blogs.msdn.com/suzcook/archive/2003/06/13/57180.aspx