Question

I am writing a class library in C# for working with matrices, and am currently working on a subclass of Matrix called ComplexMatrix. The Matrix base class works with values of the Int32 data type (a more advanced version uses Double), and the ComplexMatrix of the System.Numerics.Complex structure (.NET 4).

For the base class, I overrode ToString() as:

| 1 2 |
| 3 4 |    printed as    {{1,2}{3,4}}

The System.Numerics.Complex structure overrides ToString() in the form:

a+bi    printed as    (a,b)    where a is real and b is imaginary

When overriding ToString in ComplexMatrix, I simply used the method:

public override string ToString()
{
    return base.ToString();
}

Unfortunately, for a complex matrix, the following occurred:

| 1+1i 1+2i |
| 2+1i 2+2i |   printed as   {{0,0}{0,0}} rather than {{(1,1),(1,2)}{(2,1)(2,2)}}

The original ToString() code I wrote for the Matrix class is:

public override string ToString()
{
    StringBuilder matrixString = new StringBuilder();
    string comma = "";

    matrixString.Append("{");
    for (int i = 0; i < this.Rows; i++)
    {
        matrixString.Append("{");
        for (int j = 0; j < this.Columns; j++)
        {
            if (j == 0) comma = "";
            else comma = ",";

            matrixString.Append(comma + this.Elements[i, j].ToString());
        }
        matrixString.Append("}");
    }
    matrixString.Append("}");
    return matrixString.ToString();
}

In the above code:

• this.Elements Property: in the Matrix class, this is a 2-dimensional array of Int32 type (Double in a newer, more advanced version); it is of System.Numerics.Complex type in ComplexMatrix
• this.Rows, this.Columns Properties: the number of rows and columns respectively of the matrix

Several questions I have are:

• When ToString is called on a ComplexMatrix instance, and calls the base ToString() method, is an attempted type conversion taking place from Complex to Int32?
• As the ComplexMatrix Elements property (Complex[,] type) is hiding the base class Elements property (Int32[,] type), the new keyword is required?
• Is the “this” kwyword being seen as a Matrix type rather than ComplexMatrix?

Answer 1

I think your problem is the following:
The Elements property is not marked virtual in the Matrix class. The Elements property in the ComplexMatrix class hides the Elements property of the Matrix class. Therefore polymorphism isn’t working and the ToString method in Matrix accesses Matrix.Elements and not ComplexMatrix.Elements. But because Elements is a property and you want to change the type of the property, you can’t use virtual anyway.

To fix the problem, you should do something like this:

• Create a generic base class that wants the type of the matrix values as generic parameter
• Create a SimpleMatrix class that inherits from this base class and passes int as generic parameter.
• Create a ComplexMatrix class that inherits from this base class and passes Complex as generic parameter.