CLR vs JIT

You compile your code to IL which gets executed and compiled to machine code during runtime, this is what's called JIT.

Edit, to flesh out the answer some more (still overly simplified):

When you compile your C# code in visual studio it gets turned into IL that the CLR understands, the IL is the same for all languages running on top of the CLR (which is what enables the .NET runtime to use several languages and inter-op between them easily).

During runtime the IL is interpreted into machine code (which is specific to the architecture you're on) and then it's executed. This process is called Just In Time compilation or JIT for short. Only the IL that is needed is transformed into machine code (and only once, it's "cached" once it's compiled into machinecode), just in time before it's executed, hence the name JIT.

This is what it would look like for C#

C# Code > C# Compiler > IL > .NET Runtime > JIT Compiler > Machinecode > Execution

And this is what it would look like for VB

VB Code > VB Compiler > IL > .NET Runtime > JIT Compiler > Machinecode > Execution

And as you can see only the two first steps are unique to each language, and everything after it's been turned into IL is the same which is, as I said before, the reason you can run several different languages on top of .NET

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The JIT is one aspect of the CLR.

Specifically it is the part responsible for changing CIL (hereafter called IL) produced by the original language's compiler (csc.exe for Microsoft c# for example) into machine code native to the current processor (and architecture that it exposes in the current process, for example 32/64bit). If the assembly in question was ngen'd then the the JIT process is completely unnecessary and the CLR will run this code just fine without it.

Before a method is used which has not yet been converted from the intermediate representation it is the JIT's responsibility to convert it.
Exactly when the JIT will kick in is implementation specific, and subject to change. However the CLR design mandates that the JIT happens before the relevant code executes, JVMs in contrast would be free to interpret the code for a while while a separate thread creates a machine code representation.
The 'normal' CLR uses a pre-JIT stub approach where by methods are JIT compiled only as they are used. This involves having the initial native method stub be an indirection to instruct the JIT to compile the method then modify the original call to skip past the initial stub. The current compact edition instead compiles all methods on a type when it is loaded.

To address the addition of Generics.

This was the last major change to the IL specification and JIT in terms of its semantics as opposed to its internal implementation details.

Several new IL instructions were added, and more meta data options were provided for instrumenting types and members. Constraints were added at the IL level as well.

When the JIT compiles a method which has generic arguments (either explicitly or implicitly through the containing class) it may set up different code paths (machine code instructions) for each type used. In practice the JIT uses a shared implementation for all reference types since variables for these will exhibit the same semantics and occupy the same space (IntPtr.Size).

Each value type will get specific code generated for it, dealing with the reduced / increased size of the variables on the stack/heap is a major reason for this. Also by emitting the constrained opcode before method calls many invocations on non reference types need not box the value to call the method (this optimization is used in non generic cases as well). This also allows the default<T> behaviour to be correctly handled and for comparisons to null to be stripped out as no ops (always false) when a non Nullable value type is used.

If an attempt is made at runtime to create an instance of a generic type via reflection then the type parameters will be validated by the runtime to ensure they pass any constraints. This does not directly affect the JIT unless this is used within the type system (unlikely though possible).

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As Jon Skeet says, JIT is part of the CLR. Basically this is what is happening under the hood:

Your source code is compiled into a byte code know as the common intermediate language (CIL). Metadata from every class and every methods (and every other thing :O) is included in the PE header of the resulting executable (be it a dll or an exe). If you're producing an executable the PE Header also includes a conventional bootstrapper which is in charge of loading the CLR (Common language runtime) when you execute you executable.

Now, when you execute:

The bootstraper initializes the CLR (mainly by loading the mscorlib assembly) and instructs it to execute your assembly. The CLR executes your main entry. Now, classes have a vector table which hold the addresses of the method functions, so that when you call MyMethod, this table is searched and then a corresponding call to the address is made. Upon start ALL entries for all tables have the address of the JIT compiler. When a call to one of such method is made, the JIT is invoked instead of the actual method and takes control. The JIT then compiles the CIL code into actual assembly code for the appropiate architecture. Once the code is compiled the JIT goes into the method vector table and replaces the address with the one of the compiled code, so that every subsequent call no longer invokes the JIT. Finally, the JIT handles the execution to the compiled code. If you call another method which haven't yet being compiled then go back to 4... and so on...

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The JIT is basically part of the CLR. The garbage collector is another. Quite where you put interop responsibilities etc is another matter, and one where I'm hugely underqualified to comment :)

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I know the thread is pretty old, but I thought I might put in the picture that made me understand JIT. It's from the excellent book CLR via C# by Jeffrey Ritcher. In the picture, the metadata he is talking about is the metadata emitted in the assembly header where all information about types in the assembly is stored:

https://i.stack.imgur.com/LXVmH.png

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1)while compiling the .net program,.net program code is converted into Intermediate Language(IL) code

2)upon executing the program the Intermediate language code is converted into operating system Native code as and when a method is called; this is called JIT (Just in Time) compilation.

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Common Language Runtime(CLR) is interpreter while Just In Time(JIT) is compiler in .Net Framework.

2.JIT is the internal compiler of .NET which takes MicroSoft Intermediate Code Language (MSICL) code from CLR and executes it to machine specific instructions whereas CLR works as an engine its main task is to provide MSICL code to JIT to ensure that code is fully compiled as per machine specification.

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