Multidimensional Array Casting Runtime Error ---> unable to cast object of type 'System.Object[,]' to type 'System.String

Question

I am having trouble with what should be a simple cast of a multidimensional object array (object[,]) to a new data type (string[,]) for logging purposes. The format is a dynamic two dimensional array that has many columns and rows, but doesn't fit nicely into one of the generic collection objects offered in the framework. I would just strong type it as a string[,] all the way through the process, but I need the flexibility of the object array, because in some instances I will need to use different data types.

private List<KeyValuePair<string, object>> _dataList = new List<KeyValuePair<string, object>>();
private object[,] _dataArray;

public List<KeyValuePair<string, object>> RetrieveHistoricalData()
{

...

//Calling Method (for explaination and context purposes)

_log.Log ("\r\nRetrieveHistoricalData", "_dataList.Count: " + _dataList.Count);
_dataList.ForEach(dli => _log.Log ("\r\nRetrieveHistoricalData", "_dataList: " 
    + dli.Key + ((object[,])dli.Value)
    .CastTwoDimensionalArray<string>()
    .TwoDimensionalArrayToString()));

...

}

... Added An Extension method based on Jon Skeet's suggestions ...

internal static T[,] CastTwoDimensionalArray<T>(this object[,] dataArray)
{
    int rows = dataArray.GetLength(0);
    int columns = dataArray.GetLength(1);
    T[,] returnDataArray = new T[rows, columns];
    for (int row = 0; row < rows; row++)
    {
        for (int column = 0; column < columns; column++)
        {
            returnDataArray[row, column] =
                      (T)Convert.ChangeType(dataArray[row, column], typeof(T));
        }
    }
    return returnDataArray;
}

... Here's my own addition (only included because it is in the line I am executing) ...

internal static string TwoDimensionalArrayToString<T>(this T[,] dataArray)
{
    int rows = dataArray.GetLength(0);
    int columns = dataArray.GetLength(1);
    string returnString = "";
    for (int row = 0; row < rows; row++)
    {
        for (int column = 0; column < columns; column++)
        {
            returnString = returnString + "[" + row + "," + column + "] =>" + dataArray[row,column]+ " ;  ";
        }
    }

    return returnString;
}

I have edited the code above from the first post, but I am still receiving a System.InvalidCastException when trying to convert a System.Double to a System.String in the Generic extension method. I am working on a simple way to add some exceptions through type reflection to remove the remaining issue.

Thanks.

Thanks, but I still can't really tell what the type of the object is that you insert into _dataList :) That would allow us to inspect why your cast to string[,] is failing. — Ameen
– Ameen, Commented Jan 2, 2013 at 17:04
Ameen, I actually misstated the object in the original post. I have now corrected it to show it was actually an object[,] to begin with, not a string[,]. — identityCrisis
– identityCrisis, Commented Jan 2, 2013 at 17:11

Jon Skeet · Accepted Answer · 2013-01-02 17:37:48Z

2

EDIT: You can only cast from object[,] to string[,] if the array involved really is a string[,] originally. For example, this is fine:

object[,] o = new string[,]
{
    { "x", "y" },
    { "a", "b" }
};
string[,] x = (string[,]) o;

... but this isn't:

object[,] o = new object[,]
{
    { "x", "y" },
    { "a", "b" }
};
string[,] x = (string[,]) o; // Bang!

Even though each element of the array is a string, it still isn't a string array. In the latter case, you'll need to write your own code to create a new array object and perform the element-wise conversion. For example, here's a generic method you can use to cast each element:

using System;

class Test
{
    static void Main()
    {
        object[,] o = new object[,]
        {
            { "x", "y" },
            { "a", "b" }
        };
        string[,] x = Cast2D<string>(o);
        Console.WriteLine(x[1, 1]); // "b"
    }

    static T[,] Cast2D<T>(object[,] input)
    {
        int rows = input.GetLength(0);
        int columns = input.GetLength(1);
        T[,] ret = new T[rows, columns];
        for (int i = 0; i < rows; i++)
        {
            for (int j = 0; j < columns; j++)
            {
                ret[i, j] = (T) input[i, j];
            }
        }
        return ret;
    }        
}

In your case you'd probably want:

object[,] array = (object[,]) financialDataObject;
string[,] financialData = Cast2D<string>(array);

edited Jan 2, 2013 at 17:37

answered Jan 2, 2013 at 17:07

Jon Skeet

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3 Comments

identityCrisis Over a year ago

Thanks for the suggestion. I actually had to go back and change my first statement where I claimed the original field was a string[,]. It was actually an object[,] for its origin. I went back and looked at other portions of my code and noticed that I am expecting to receive a myriad of different types of historical data that I will have to be convert for, so I'll probably have to write a bunch of conversion logic inside of a generic method like the one you posted above.

Jeppe Stig Nielsen Over a year ago

Jon: The "crazy" array covariance of .NET 1 seems to work fine with 2D arrays. I just ran this code successfully: string[,] arr1 = { { "Hello", "this", }, { "is", "working", }, }; object[,] arr2 = arr1; Console.WriteLine(arr2[1, 1]);

Jon Skeet Over a year ago

@JeppeStigNielsen: You're absolutely right. I swear I tried it before posting, and the compiler told me there was no such conversion available. No idea what I was doing now. Will edit.

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