Microsoft Solver Foundation Throws ArgumentNullException When Adding a Constraint

Question

I'm trying to create a schedule for a call centre where I am trying to maximize staff on high call periods and minimize on low periods. For simplicity, it would look something like:

Simple Call Centre Schedule

I have the following code:

var startTimes = BuildStartTimes();
var employees = BuildEmployees();
ConstraintSystem solver = ConstraintSystem.CreateSolver();
CspDomain wrkTime = solver.CreateIntegerSet(startTimes);
CspTerm[][] scheduleMatrix = solver.CreateVariableArray(wrkTime, "Schedule", employees.Length, startTimes.Length);

//iterate through times adding column constraints
for (int i = 0; i < startTimes.Length -1; i++)
{
    //add constraint for employees numbers
    for (int emp = 0; emp < employees.Length; emp++)
    {
        //for simplistic sake, the Ids i to 9 represent employee Ids
        scheduleMatrix[emp][i].Model.CreateIntegerSet(new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 }); 
    }

    solver.AddConstraints(
            //add constraint that the employee can be added just once
            solver.GreaterEqual(NumberOfWorkersRequiredForShift(i, employees), GetColumn(scheduleMatrix, i)),
            //employee can only list once
            solver.Unequal(GetColumn(scheduleMatrix,i))
        );
}

for (int i = 0; i < employees.Length -1; i++)
{
    solver.AddConstraints(
            //employee can only listed nine consecutive 36 times maximum 
            //4 * 15 minutes * 9 hours == 4 * 9 == 36
            solver.Equal(36,GetRow(scheduleMatrix,i))
        );
}

private static CspTerm[] GetColumn(CspTerm[][] matrix, int column)
{

    CspTerm[] slice = new CspTerm[matrix.Length];
    for (int row = 0; row < matrix.Length; row++)
        slice[row] = matrix[row][column];

    return slice;
}

private static CspTerm[] GetRow(CspTerm[][] matrix, int row)
{
    CspTerm[] slice = new CspTerm[matrix[0].Length];
    for (int col = 0; col < matrix.Length; col++)
        slice[col] = matrix[row][col];

    return slice;
}

I am getting a ArgumentNullException on the constraint of limiting the employee to 9 hours (e.g. 4 fifteen minutes section * 9 hours = 36 times).

Here's the stack trace:

at Microsoft.SolverFoundation.Solvers.ConstraintSystem.ValidateInputs(CspTerm[] inputs) at Microsoft.SolverFoundation.Solvers.ConstraintSystem.Equal(Int32 constant, CspTerm[] inputs) at CSPCallCenterDemo.Program.Main(String[] args) in c:\Users\wdniels\Documents\Visual Studio 2012\Projects\CSPCallCenterDemo\CSPCallCenterDemo\Program.cs:line 40 at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args) at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args) at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly() at System.Threading.ThreadHelper.ThreadStart_Context(Object state) at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading.ThreadHelper.ThreadStart()

Also does anyone know how I can add a constraint that ensures that the employee will work 36 consecutive 15 minute intervals?

There is an obvious typo in GetRow(). Code defensively and use slice.Length instead. Throwing in a couple of asserts that verify that the jagged array arrays all have the same length and their elements are not null never hurts either. — Hans Passant

gabba gabba · Accepted Answer · 2014-10-26T19:39:02

You get ArgumentException because of you GetRow code did't completely fill CspTerm. You have to rewrite it like this:

    private static CspTerm[] GetRow(CspTerm[][] matrix, int row)
    {
        CspTerm[] slice = new CspTerm[matrix[0].Length];
        for (int col = 0; col < matrix[0].Length; col++)
            slice[col] = matrix[row][col];

        return slice;
    }

Microsoft Solver Foundation Throws ArgumentNullException When Adding a Constraint

1 Answers