Square Integer Matrix, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Text;

namespace Rextester
{
    public class Program
    {
        
        public class SquareIntegerMatrix
        {
            private int[,] InternalRep = null;

public int Rows = 0;
            public int Columns = 0;
            private List<int> Vector = null;
            private void Zero()
            {
                for (int i = 0; i < Rows; i++)
                {
                    for (int j = 0; j < Columns; j++)
                    {
                        InternalRep[i, j] = 0;
                    }
                }
            
            }

private void FromVector()
            {
                int cnt = 0;
                for (int i = 0; i < Rows; i++)
                {
                    for (int j = 0; j < Columns; j++)
                    {
                        InternalRep[i, j] = Vector[cnt++];
                    }
                }
            }

public SquareIntegerMatrix(int rows, int columns)
            {
                if (rows != columns)
                {
                    throw new Exception("rows and columns must be equal for square matrix");
                    
                }

this.Rows = rows;
                this.Columns = columns;
                InternalRep = new int[this.Rows, this.Columns];

Zero();
            }

public SquareIntegerMatrix(int[,] RC)
            {
                int rows = RC.GetLength(0);
                int columns = RC.GetLength(1);

if (rows != columns)
                {
                    throw new Exception("rows and columns must be equal for square matrix");

}

this.Rows = rows;
                this.Columns = columns;
                InternalRep = RC;        
            }
            
            public SquareIntegerMatrix(int rows, int columns, List<int> V)
            {
                if (rows != columns)
                {
                    throw new Exception("rows and columns must be equal for square matrix");

}
                
                if (V.Count % rows != 0)
                {
                    throw new Exception("Vector does not contain even row count");
                }

Vector = V;

this.Rows = rows;
                this.Columns = columns;
                InternalRep = new int[this.Rows, this.Columns];

FromVector();
            }

public static SquareIntegerMatrix operator +(SquareIntegerMatrix a, SquareIntegerMatrix b)
            {
                SquareIntegerMatrix ret = new SquareIntegerMatrix(a.Rows, a.Columns);

for (int i = 0; i < ret.Rows; i++)
                {
                    for (int j = 0; j < ret.Columns; j++)
                    {

ret.InternalRep[i, j] = a.InternalRep[i, j] + b.InternalRep[i, j];                        
                    
                    }

}

return ret;
            }
            
            public static SquareIntegerMatrix operator *(SquareIntegerMatrix a, SquareIntegerMatrix b)
            {
                SquareIntegerMatrix ret = new SquareIntegerMatrix(a.Rows, a.Columns);

for (int i = 0; i < ret.Rows; i++)
                {
                    for (int j = 0; j < ret.Columns; j++)
                    {

for (int k = 0; k < ret.Columns; k++)
                        {
                            ret.InternalRep[i, j] += a.InternalRep[i, k] * b.InternalRep[k, j];
                        }
                    
                    }

}

return ret;
            }

public int this[int r, int c]
            {
                get
                {
                    if (!(r < Rows && c < Columns))
                    {
                        throw new Exception("rows and columns out of range of square matrix");
                    }
                    return (InternalRep[r, c]); 
                }
                set { InternalRep[r, c] = value; }
            }

public override string ToString()
            {
                StringBuilder sb = new StringBuilder();
                for (int i = 0; i < Rows; i++)
                {
                    for (int j = 0; j < Columns; j++)
                    {
                        sb.AppendFormat("{0}\t", InternalRep[i, j]);
                    }

sb.Append("\r\n");

}

return sb.ToString();
            }
        }
        
        public static void Main(string[] args)
        {
            List<int> v = new List<int> { 1, 2, 3, 4 };
            List<int> v2= new List<int> { 7, 8, 9, 10 };

SquareIntegerMatrix m = new SquareIntegerMatrix(2, 2, v);
            SquareIntegerMatrix m2 = new SquareIntegerMatrix(2, 2, v2);

SquareIntegerMatrix mm = m * m2;

Console.WriteLine(mm.ToString());
            Console.WriteLine(mm[0, 0]);

mm[0, 0] = 97;
            Console.WriteLine(mm.ToString());
            
            int[,] ir = new int[2, 2];
            
            ir[0, 0] = 1;
            ir[0, 1] = 2;
            ir[1,0] = 3;
            ir[1,1] = 4;
            
            SquareIntegerMatrix mmir = new SquareIntegerMatrix(ir);
            Console.WriteLine(mmir.ToString());
            
            SquareIntegerMatrix mma = m + m2;
            Console.WriteLine(mma.ToString());
            
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,14 sec, absolute running time: 0,09 sec, cpu time: 0,08 sec, average memory usage: 14 Mb, average nr of threads: 3