How is LINQ compiled into the CIL?

For example:

var query = from c in db.Cars select c;
foreach(Car aCar in query)

How would this translate once it is compiled? What happens behind the scenes?

Here is Solutions:

We have many solutions to this problem, But we recommend you to use the first solution because it is tested & true solution that will 100% work for you.

Solution 1

It is compiled in the following way:

  1. First, the LINQ query expression is transformed into method calls:

    public static void Main()
        var query = db.Cars.Select<Car, Car>(c => c);
        foreach (Car aCar in query)
  2. If db.Cars is of type IEnumerable<Car> (which it is for LINQ-to-Objects), then the lambda expression is turned into a separate method:

    private Car lambda0(Car c)
        return c;
    private Func<Car, Car> CachedAnonymousMethodDelegate1;
    public static void Main()
        if (CachedAnonymousMethodDelegate1 == null)
            CachedAnonymousMethodDelegate1 = new Func<Car, Car>(lambda0);
        var query = db.Cars.Select<Car, Car>(CachedAnonymousMethodDelegate1);
        foreach // ...

    In reality the method is not called lambda0 but something like <Main>b__0 (where Main is the name of the containing method). Similarly, the cached delegate is actually called CS$<>9__CachedAnonymousMethodDelegate1.

    If you are using LINQ-to-SQL, then db.Cars will be of type IQueryable<Car> and this step is very different. It would instead turn the lambda expression into an expression tree:

    public static void Main()
        var parameter = Expression.Parameter(typeof(Car), "c");
        var lambda = Expression.Lambda<Func<Car, Car>>(parameter, new ParameterExpression[] { parameter }));
        var query = db.Cars.Select<Car, Car>(lambda);
        foreach // ...
  3. The foreach loop is transformed into a try/finally block (this is the same for both):

    IEnumerator<Car> enumerator = null;
        enumerator = query.GetEnumerator();
        Car aCar;
        while (enumerator.MoveNext())
            aCar = enumerator.Current;
        if (enumerator != null)
  4. Finally, this is compiled into IL the expected way. The following is for IEnumerable<Car>:

    // Put db.Cars on the stack
    L_0016: ldloc.0 
    L_0017: callvirt instance !0 DatabaseContext::get_Cars()
    // “if” starts here
    L_001c: ldsfld Func<Car, Car> Program::CachedAnonymousMethodDelegate1
    L_0021: brtrue.s L_0034
    L_0023: ldnull 
    L_0024: ldftn Car Program::lambda0(Car)
    L_002a: newobj instance void Func<Car, Car>::.ctor(object, native int)
    L_002f: stsfld Func<Car, Car> Program::CachedAnonymousMethodDelegate1
    // Put the delegate for “c => c” on the stack
    L_0034: ldsfld Func<Car, Car> Program::CachedAnonymousMethodDelegate1
    // Call to Enumerable.Select()
    L_0039: call IEnumerable<!!1> Enumerable::Select<Car, Car>(IEnumerable<!!0>, Func<!!0, !!1>)
    L_003e: stloc.1
    // “try” block starts here
    L_003f: ldloc.1 
    L_0040: callvirt instance IEnumerator<!0> IEnumerable<Car>::GetEnumerator()
    L_0045: stloc.3
    // “while” inside try block starts here
    L_0046: br.s L_005a
    L_0048: ldloc.3   // body of while starts here
    L_0049: callvirt instance !0 IEnumerator<Car>::get_Current()
    L_004e: stloc.2 
    L_004f: ldloc.2 
    L_0050: ldfld string Car::Name
    L_0055: call void Console::WriteLine(string)
    L_005a: ldloc.3   // while condition starts here
    L_005b: callvirt instance bool IEnumerator::MoveNext()
    L_0060: brtrue.s L_0048  // end of while
    L_0062: leave.s L_006e   // end of try
    // “finally” block starts here
    L_0064: ldloc.3 
    L_0065: brfalse.s L_006d
    L_0067: ldloc.3 
    L_0068: callvirt instance void IDisposable::Dispose()
    L_006d: endfinally 

    The compiled code for the IQueryable<Car> version is also as expected. Here is the important part that is different from the above (the local variables will have different offsets and names now, but let’s disregard that):

    // typeof(Car)
    L_0021: ldtoken Car
    L_0026: call Type Type::GetTypeFromHandle(RuntimeTypeHandle)
    // Expression.Parameter(typeof(Car), "c")
    L_002b: ldstr "c"
    L_0030: call ParameterExpression Expression::Parameter(Type, string)
    L_0035: stloc.3 
    // Expression.Lambda(...)
    L_0036: ldloc.3 
    L_0037: ldc.i4.1           // var paramArray = new ParameterExpression[1]
    L_0038: newarr ParameterExpression
    L_003d: stloc.s paramArray
    L_003f: ldloc.s paramArray
    L_0041: ldc.i4.0                    // paramArray[0] = parameter;
    L_0042: ldloc.3 
    L_0043: stelem.ref 
    L_0044: ldloc.s paramArray
    L_0046: call Expression<!!0> Expression::Lambda<Func<Car, Car>>(Expression, ParameterExpression[])
    // var query = Queryable.Select(...);
    L_004b: call IQueryable<!!1> Queryable::Select<Car, Car>(IQueryable<!!0>, Expression<Func<!!0, !!1>>)
    L_0050: stloc.1 

Solution 2

You should compile it and run ildasm against the resulting executable to find out.

Note: Use and implement solution 1 because this method fully tested our system.
Thank you 🙂

All methods was sourced from or, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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