Cannot apply indexing with [] to an expression of type `object'

Question

Here is my code: An ArrayList of ArrayList that returns a float:

public ArrayList walls=new ArrayList(); 

public void Start()
{
    walls[0] = ReturnInArrayList(279,275,0,0,90);
    walls[1] = ReturnInArrayList(62,275,0,0,0);
    walls[2] = ReturnInArrayList(62,275,62,0,90);
    walls[3] = ReturnInArrayList(217,275,62,-62,0);
    walls[4] = ReturnInArrayList(62,275,279,0,90);
    walls[5] = ReturnInArrayList(41,275,279,0,0);
    walls[6] = ReturnInArrayList(279,275,320,0,9);
    walls[7] = ReturnInArrayList(320,275,0,-279,0); 

    for (int i = 0; i < walls.Length; i++) {
        float a = (float)walls[i][0];
        float b = (float)walls[i][1];
        float c = (float)walls[i][2];
        float d = (float)walls[i][3];
        float e = (float)walls[i][4];
    }
}

ArrayList ReturnInArrayList(float a,float b,float c, float d, float e)
{
    ArrayList arrayList = new ArrayList();
    arrayList.Add(a);
    arrayList.Add(b);
    arrayList.Add(c);
    arrayList.Add(d);
    arrayList.Add(e);
    return arrayList;
}

It gives me the following error:

error CS0021: Cannot apply indexing with [] to an expression of type `object'

I already did the casting, what is wrong? :(

Does the compiler provide a hint as to where the error might be? — HABO, Dec 22 '12 at 16:26
I don't see where your code applies indexing to an expression of type object. Do we get a clue? — Jonathan Wood, Dec 22 '12 at 16:27
Yes, it provides hint, sorry: All these lines: " float a=(float)paredes[i][0]; float b=(float)paredes[i][1]; float c=(float)paredes[i][2]; float d=(float)paredes[i][3]; float e=(float)paredes[i][4]; " — JamesB, Dec 22 '12 at 17:13

score 14 · Accepted Answer · answered Dec 22 '12 at 16:28

The problem is that paredes[i] returns an object which is the return type of the ArrayList indexer. You need to cast this to an ArrayList to be able to access it:

float a= (float)((ArrayList)paredes[i])[0];

A better solution though is to use generics and populate a List<float> instead:

List<float> RetornaEmList(float a,float b,float c, float d, float e)
{
    return new List<float> { a, b, c, d, e };
}

then paredes is a List<List<float>> and your accessor can be changed to:

float a = paredes[i][0];

Thanks so much. I was iterating over the rows in a datatable with foreach (var row in dt.Rows) {} and it wasn't recognizing the typical row["ID"]. Once I changed var to DataRow everything worked. — mdegges, Jul 20 '18 at 22:03

Scott Chapman · Answer 2 · 2012-12-22T16:43:25.133

ArrayList stores objects without limiting what type those objects are. When you access the objects stored in an ArrayList, the compiler doesn't know what type they are, so it just gives you type object.

You're storing an ArrayList of float in your outer ArrayList. Since you're always storing floats, it would be better to use a List<float> for the inner list, and a List<List<float>> for the outer list. This way you won't have to type cast from object:

using System.Collections.Generic;

public List<List<float>> paredes = new List<List<float>>();   


Start() {
    paredes[0]=RetornaEmList(279,275,0,0,90);
    paredes[1]=RetornaEmList(62,275,0,0,0);
    paredes[2]=RetornaEmList(62,275,62,0,90);
    paredes[3]=RetornaEmList(217,275,62,-62,0);
    paredes[4]=RetornaEmList(62,275,279,0,90);
    paredes[5]=RetornaEmList(41,275,279,0,0);
    paredes[6]=RetornaEmList(279,275,320,0,9);
    paredes[7]=RetornaEmList(320,275,0,-279,0);    



    for (int i=0;i<paredes.Length;i++)
    {           
        float a=paredes[i][0];
        float b=paredes[i][1];
        float c=paredes[i][2];
        float d=paredes[i][3];
        float e=paredes[i][4];                   
    }
}

List<float> RetornaEmList(float a,float b,float c, float d, float e)
{
    return new List<float> { a, b, c, d, e };
}

Since the inner list always has 5 floats, you could also use a float[] instead of a List<float>

If you just want to make the code work without moving from ArrayList to List, you need an additional cast:

float a = (float)((ArrayList)paredes[i])[0];

But it's a lot cleaner just to use List<float> instead.

score 1 · Answer 3 · edited Dec 22 '12 at 16:38

1

Where are you doing the casting?

I would say it must be (did not try with a compiler):

for (int i = 0; i < paredes.Length; i++)
{           
    float a=(float)((ArrayList)paredes[i])[0];
    ...                   
}

Did you consider using the generic collections instead?

public List<List<float>> paredes = new List<List<float>>();

edited Dec 22 '12 at 16:38

Olivier Jacot-Descombes

104,806
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answered Dec 22 '12 at 16:28

CSharper

524
3
9

Kundan Singh Chouhan · Answer 4 · 2012-12-22T16:47:48.897

0

Code should be like this :

//Adding List

paredes.Add(RetornaEmArrayList(279,275,0,0,90));
paredes.Add(RetornaEmArrayList(62,275,0,0,0));
paredes.Add(RetornaEmArrayList(62,275,62,0,90));
paredes.Add(RetornaEmArrayList(217,275,62,-62,0));
paredes.Add(RetornaEmArrayList(62,275,279,0,90));
paredes.Add(RetornaEmArrayList(41,275,279,0,0));
paredes.Add(RetornaEmArrayList(279,275,320,0,9));
paredes.Add(RetornaEmArrayList(320,275,0,-279,0));


//Traversing List

foreach(ArrayList item in paredes)
{
    float a=(float)item[0];
    float b=(float)item[1];
    float c=(float)item[2];
    float d=(float)item[3];
    float e=(float)item[4];
}

edited Dec 22 '12 at 16:47

answered Dec 22 '12 at 16:32

Kundan Singh Chouhan

13,952
4
27
32

1

I believe that would cause the same error, because `item` is an `object`. – svick Dec 22 '12 at 16:33
@svick, item is a list and item[0] is a float element. – Kundan Singh Chouhan Dec 22 '12 at 16:35
1

Yeah, but the compiler doesn't know that, which is why the original code caused the error, and your code doesn't fix that. – svick Dec 22 '12 at 16:36
As svick points out, this won't compile since `item` will be typed as `object`. It should if you replace `var` with `ArrayList` though. – Lee Dec 22 '12 at 16:40

Cannot apply indexing with [] to an expression of type `object'

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