C#

namespace PopData
{
public partial class FormPopData : Form    
{
const int SIZE = 39;        
const int NEW_SIZE = 39;        
int[] population = new int[SIZE];
int[] pop_diff = new int[NEW_SIZE];      
int index;                
public FormPopData()        
{            
InitializeComponent();        
}        
private void buttonCalculate_Click(object sender, EventArgs e)
{
//Open the USPopulation.txt file.
StreamReader inputFile = File.OpenText("USPopulation.txt");
//Read the data into the ListBox.
List<int> PopulationList = new List<int>();
int year = 1950;
while (!inputFile.EndOfStream)
{
PopulationList.Add(int.Parse(inputFile.ReadLine()));
}
//Close the file.
inputFile.Close();
int numberofyears = PopulationList.Count();
int average = Average(PopulationList);
textBoxAverage.Text = average.ToString();
int greatest = year + GreatestIncrease(PopulationList);
textBoxGreatest.Text = greatest.ToString();
int least = year + LeastIncrease(PopulationList);
textBoxLeast.Text = least.ToString();
}
}    
private int diff(int[] population)        
{           
//const int New_Size = 39;            
int count = 0;            
//int difference;            
try            
{                
pop_diff = new int[NEW_SIZE];                
for (int index = 1; index <= population.Length - 1; index++)                
{                   
pop_diff[count] = population[index] - population[index - 1] / populationindex - >1];                  
count += 1;              
}           
}          
catch (Exception ex)          
{              
//Display an error message.             
MessageBox.Show(ex.Message);          
}          
return pop_diff[count];     
}       
private int Average(List<int> PopulationList)      
{         
int amountofchange = 0;         
int changeinyears = 0;         
int thechange = 0;          
int currentyear = 0;          
//int numberofyears = 39;          
for (int index = 0; index < PopulationList.Count; index++)          
{              
thechange += PopulationList[index];             
currentyear = index;          
}         
amountofchange = (int)thechange / PopulationList.Count;          
return changeinyears;     
}
private int GreatestIncrease(List<int> PopulationList)       
{         
int amountofchange = 0;       
int changeinyears = 0;         
int thechange = 0;          
int currentyear = 0;          
for (int index = 1; index < PopulationList.Count(); index++)          
{              
thechange = PopulationList[index] - PopulationList[index - 1];            
currentyear = index;            
if (thechange > amountofchange)               
{                 
amountofchange = thechange;                
changeinyears = index;              
}        
}        
return changeinyears;    
}    
private int LeastIncrease(List<int> PopulationList)    
{          
int firstitem = PopulationList.First();         
int lastitem = PopulationList.Last();          
int amountofchange = (lastitem - firstitem);         
int changeinyears = 0;         
int thechange = 0;        
int currentyear = 0;        
for (int index = 1; index < PopulationList.Count(); index++)        
{              
thechange = PopulationList[index] - PopulationList[index - 1];             
currentyear = index;         
if (thechange < amountofchange)            
{               
amountofchange = thechange;             
changeinyears = index;           
}       
}       
return changeinyears;
}

int? tmp = 10;
tmp = null;
Console.WriteLine(tmp.ToString()); // works fine

Console.WriteLine(tmp.GetType()); // NullReferenceException