Recently I received a NullReference-Exception when I called another method from a foreign component. Fortunately I had the source code of this component and I found the following code:
[code language=”csharp”]
public class AgeValidator
{
public Dictionary Config { get; set; }

public bool Validate(int nAge)
{
int nMinAge = FindValue(“MinAge”);

return nAge > nMinAge;
}

private int FindValue(string strParameter)
{
int nValue = -1;

if(this.Config.ContainsKey(strParameter)) nValue = this.Config[strParameter];

return nValue;
}
}
[/code]

There are several problems with this code. First, there are no preconditions which checks if the object is in a valid state or the parameters are valid. This cause to behaviour which isn’t intended. The problem with a not available configuration could already detect in the Validate method. The Find method could check if the parameter is valid, if not it doesn’t make any sense to continue. The FindValue method doesn’t follow the Fail Fast principle. Jim Shore wrote an excellent article about the Fail Fast principle. After a refactoring the code looks like that:
[code language=”csharp”]
public class AgeValidator
{
public Dictionary Config { get; set; }

public bool Validate(int nAge)
{
if(this.Config == null || this.Config.Keys.Count == 0) throw new InvalidOperationException(“No valid configuration available.”);

int nMinAge = FindValue(“MinAge”);

return nAge > nMinAge;
}

private int FindValue(string strParameter)
{
if(string.IsNullOrEmpty(strParameter)) throw new ArgumentNullException(“strParameter”);
if(!this.Config.ContainsKey(strParameter)) throw new ArgumentException(string.Format(“No configuration for the key ‘{0}’ found.”, strParameter));

return this.Config[strParameter];
}
}
[/code]

What I advise junior software developers is to follow the Fail Fast principle, disciplined use of preconditions and if it adequate the use of postconditions. But all these advices don’t replacing unit testing.