How to create multi-dimensional array using Reflection.Emit

Question

I want to create a multi-dimensional array using Reflection.Emit and set it's element. Like the following C# code:

int[,] nums = new int[2, 2];
nums[1, 1] = 2;

And turn into IL code:

IL_0000: nop
IL_0001: ldc.i4.2
IL_0002: ldc.i4.2
IL_0003: newobj instance void int32[0..., 0...]::.ctor(int32, int32)
IL_0008: stloc.0
IL_0009: ldloc.0
IL_000a: ldc.i4.1
IL_000b: ldc.i4.1
IL_000c: ldc.i4.2
IL_000d: call instance void int32[0..., 0...]::Set(int32, int32, int32)

The IL code to create array:

newobj instance void int32[0..., 0...]::.ctor(int32, int32)

And the IL code to set the array's element:

call instance void int32[0..., 0...]::Set(int32, int32, int32)

What kind of IL Generator.Emit() code corresponding to those two IL sentence?

poke · Accepted Answer · 2016-03-27 18:40:22Z

3

You can almost translate that IL verbally:

il.Emit(OpCodes.Ldc_I4_2);
il.Emit(OpCodes.Ldc_I4_2);

var constructor = typeof(int[,]).GetConstructor(new Type[]{ typeof(int), typeof(int) });
il.Emit(OpCodes.Newobj, constructor);
il.Emit(OpCodes.Stloc_0);
il.Emit(OpCodes.Ldloc_0);
il.Emit(OpCodes.Ldc_I4_1);
il.Emit(OpCodes.Ldc_I4_1);
il.Emit(OpCodes.Ldc_I4_2);

var setMethod = typeof(int[,]).GetMethod("Set");
il.Emit(OpCodes.Call, setMethod);

Of course, you need to use reflection to actually get the ConstructorInfo and the MethodInfo object you need for the Newobj and Call codes.

answered Mar 27, 2016 at 18:40

poke

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2 Comments

nineveh.Y Over a year ago

thank you, I wrote [ , ] as [ ][ ] at first and got wrong IL code, now I see

poke Over a year ago

[,] and [][] refer to two different types of arrays, see this question for more information about the differences.

Yacoub Massad · Accepted Answer · 2016-03-27 18:46:15Z

Here is an example:

DynamicMethod method =
    new DynamicMethod("Test" , typeof(int[,]), new Type[]{});

var generator = method.GetILGenerator();

//get the constructor that takes in 2 integers (the dimensions of the array)
var constructor = typeof (int[,])
    .GetConstructor(new {typeof (int), typeof (int)});

//get the Set method that takes in 3 integers; 2 indexes and the value 
var set_method = typeof(int[,])
    .GetMethod("Set", new[] { typeof(int), typeof(int), typeof(int) });

var local = generator.DeclareLocal(typeof (int[,])); //local variable to reference the array

generator.Emit(OpCodes.Ldc_I4_2);
generator.Emit(OpCodes.Ldc_I4_2);
generator.Emit(OpCodes.Newobj, constructor); //invoke the constructor to create the array
generator.Emit(OpCodes.Stloc, local);
generator.Emit(OpCodes.Ldloc, local);
generator.Emit(OpCodes.Ldc_I4_1);
generator.Emit(OpCodes.Ldc_I4_1);
generator.Emit(OpCodes.Ldc_I4_2);
generator.Emit(OpCodes.Call, set_method); //call the Set method to set the value
generator.Emit(OpCodes.Ldloc, local);
generator.Emit(OpCodes.Ret);

var result_method = (Func<int[,]>)method.CreateDelegate(typeof (Func<int[,]>));

var result = result_method(); //returns the array

This example creates a dynamic method that creates the array, fills the value in [1,1], and then returns that array.

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