Convert Dynamic 2D ArrayList to 2D Float Array

public float[,] GetFoo()
{
string inputFileName = "MyData.txt";
//// Values in file MyData.txt
//3 -2 1 0 4
//-2 5 -1 1 7
//1 -1 -11 -2 2
//0 1 -2 12 3
//string outputFileName = "MyTest.txt";
char[] separator = { '\t'};
FileStream inFile = new FileStream(inputFileName, FileMode.Open);
StreamReader sr = new StreamReader(inFile);
//FileStream outFile = new FileStream(outputFileName, FileMode.Create);
//StreamWriter sw = new StreamWriter(outFile);
//System.Collections.ArrayList al = new ArrayList();
ArrayList al = new ArrayList();
int counter = 0;
string a = " ";
while (sr.Peek() != -1)
{
string strLine = sr.ReadLine();
strLine = strLine.Trim(' ');
string[] aValues = strLine.Split(separator);
int NumOfCols = aValues.Length;
for (int j = 0; j < NumOfCols; j++)
{
ArrayList alCol = new ArrayList();
al.Add(alCol);
float value;
value = Convert.ToSingle(aValues[j]);
((ArrayList) al[counter]).Add(value);
// This line is fine and displays the contents of the 2D array list
a += Convert.ToString(((ArrayList)al[counter])[j]) + '\t';
}
counter += 1;
a += '\n';
}
sr.Close();
//sw.Close();
MessageBox.Show(a, "Array A Test");
// This is the line with the problem! See message below.
return (float[,])al.ToArray(typeof(float[][]));
// Window Message
//"InvalidCastException" was unhandled.
//At least one element in the source array could not be cast down to the destination array type.
//When casting from a number, the value must be a number less than infinity.
}