namespace Exam
{
    public partial class Form1 : Form
    {
        public Form1()
        {
         
       InitializeComponent();
        }
    
Private void btnCalculate_Click (object sender, System.EvenArgs e)
	{	
		try
        {
            If   (IsValidData());	
			{
                int Numofyrs = Convert.ToInt32(txtnumYrs);
                Decimal discountRate = Convert.ToDecimal(txtDuiscountRate);
                Decimal yrlyCashFlow = Convert.ToDecimal(txtYrlyCashFlows);
                Decimal onetimecosts = Convert.ToDecimal(txtOneTimeCosts);
                Decimal yrlyrecurrcosts = Convert.ToDecimal(txtRecurCosts);
                
                npv = CalcNPV(discountRate,yrlyCashFlow,onetimecosts,yrlyrecurrcosts,Numofyrs);
                roi = CalcROI(npv,onetimecosts,yrlyrecurrcosts,Numofyrs,discountRate);
                bep = CalcBEP(discountRate,yrlyCashFlow,yrlyrecurrcosts,onetimecosts,Numofyrs);

                txtOverallNPV.Text=npv.tostring();
                txtOverallROI.Text=roi.tostring();
                txtBrkEven.Text=bep.tostring();

            }   
    }
    
        catch (Exception ex)
                 {
                     MessageBox.Show(ex->Message + "\n\n" + 
                         ex->GetType()->ToString() + "\n" +
                         ex->StackTrace, "Exception");
                 }
             }

    
    

                public bool IsValidData()
                {
                return
                    
                    IsPresent(txtmNuYears, "Num Yrs") &&
                    IsInt32(txtNumYears, "Num Yrs") &&
                    IsWithinRange(txtNumYears, "Num Yrs", 1, 10)&&

                    IsPresent(txtDuiscountRate, "d rate")&&
                    IsDecimal(txtDuiscountRate, "d rate")&&
                    IsRange(txtDuiscountRate, "d rate")&&

                    IsPresent(txtYrlyCashFlows, "ycf")&&
                    IsDecimal(txtYrlyCashFlows, "ycf")&&
                    IsRange(txtYrlyCashFlows, "ycf")&&

                    IsPresent(txtOneTimeCosts, "otc")&&
                    IsDecimal(txtOneTimeCosts, "otc")&&
                    IsRange(txtOneTimeCosts, "otc")&&

                    IsPresent(txtRecurCosts, "rec cost")&&
                    IsDecimal(txtRecurCosts, "rec cost")&&
                    IsRange(txtRecurCosts, "rec cost");
                }
    
           
public bool IsPresent(TextBox textBox, String name)
            {
                if (textBox.Text = "")
                {
                    MessageBox.Show(name + " is a required field.", "Entry Error");
                    textBox.Focus();
                    return false;
                }
                return true;
            }
               
public bool IsInt32(TextBox textBox, String name)
            {
                try
                {
                    Convert.ToInt32(textBox.Text);
                    return true;
                }
                catch (FormatException)
                {
                    MessageBox.Show(name + " must be an integer.", "Entry Error");
                    textBox.Focus();
                    return false;
                }
            }
                
public bool IsWithinRange(TextBox textBox, String name, double min, double max)
            {
                double number = Convert.ToDouble(textBox.Text);
                if (number < min || number > max)
                {
                    MessageBox.Show(name + " must be between " + min
                        + " and " + max + ".", "Entry Error");
                    textBox.Focus();
                    return false;
                }
                return true;
            }


 private decimal CalcNPV(decimal rate, decimal cashflow, decimal onetime, decimal recurring, int year)
        {
            double sum = 0;
            do
            {
                sum += ((Convert.ToDouble(cashflow - recurring)) / Math.Pow(Convert.ToDouble(1 + rate), Convert.ToDouble(year)));
                year--;
            } while (year > 0);

            return Convert.ToDecimal(sum - Convert.ToDouble(onetime));
        }

        private decimal CalcROI(decimal npv, decimal recurring, decimal onetime, int year, decimal rate)
        {
             double sum = 0;
             do
             {
                 sum += ((Convert.ToDouble(recurring)) / Math.Pow(Convert.ToDouble(1 + rate), Convert.ToDouble(year)));
                 year--;
             } while (year > 0);

             return Convert.ToDecimal(Convert.ToDouble(npv) / (Convert.ToDouble(onetime) + sum));
        }

        private int CalcBEP(decimal rate, decimal cashflow, decimal recurring, decimal onetime, int year)
        {
            double sum1 = 0;
            double sum2 = 0;
            int i = 0;
            do
            {
                sum1 += ((Convert.ToDouble(cashflow)) / Math.Pow(Convert.ToDouble(1 + rate), Convert.ToDouble(year)));
                sum2 += ((Convert.ToDouble(recurring)) / Math.Pow(Convert.ToDouble(1 + rate), Convert.ToDouble(year)));
                i++;

                if ((sum1 - sum2) - Convert.ToDouble(onetime) > 0)
                    break;
                else if (i == 100)
                    break;
                else
                    onetime -= Convert.ToDecimal((sum1 - sum2));

            }while(i >= 0);

            return i;
}}}

the error is in "Private void btnCalculate_Click (object sender, System.EvenArgs e)"

it says invalid token void in class, struct, or interface member decleration

the error is in "Private void btnCalculate_Click (object sender, System.EvenArgs e)"

it says invalid token void in class, struct, or interface member decleration

That error means that there is a unknown token present, which is caused by the compiler not knowing what an object is. In this case, the second paramater is misspelled. It should say System.EventArgs instead of System.EvenArgs.