public CalcCommand(CalcViewModel viewModel)
 {
     this.viewModel = viewModel;
 }
 public DeleteCharCommand(CalcViewModel viewModel)
 {
     this.viewModel = viewModel;
 }
 public OperationCommand(CalcViewModel viewModel)
 {
     this.viewModel = viewModel;
 }
 public AddCharCommand(CalcViewModel viewModel)
 {
     this.viewModel = viewModel;
 }