Using Default values for Nullable Variables is a Bad Practice — .NET Core

All nullable variables, or class members (excluding collections) should have their value set to NULL unless a value is available. Setting a default value to a nullable is a bad practice.

What do I mean? well, see the code blocks below:

// Below is the wrong way to consume a value

Decimal myValue = GetDummyValue() ?? 0m;
DoSomthing(myValue);

// Below is the correct way to comsume a value

Decimal? myValue = GetDummyValue();

if (myValue.HasValue) 
{
  DoSomthing(myValue);
}

Reason: a NULL is itself a value to indicate that the required value is not available to consume. Setting a default value muddies the water and creates confusion.

Why so? so here is my story …

Just recently while refactoring an internal project, an API started producing strange results.

There was not much changed during refactoring, just a few lines of code where a default DateTime type value was being set to a nullable variable.

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the code initially looked something like below:

public ActionResult<double> GetUserActivity()
{
  DateTime expiration = _userContext?.GetSessionExpiration() ?? DateTime.MinValue;
  if(expiration == DateTime.MinValue) { return this.Ok(999.9); }
  double remaining = (expiration - DateTime.Now).TotalSeconds;
  return this.Ok(remaining);
}

After the refactor, the code was changed to something like the below:

public ActionResult<double> GetUserActivity()
{
  double expirationSeconds = null; 
  var expiration = this._userContext?.GetSessionExpiration();

  if (expiration != null)
  {
    var expirationSeconds = (expiration- DateTime.Now).TotalSeconds;
  }

  return this.Ok(expirationSeconds);
}

It was mind-boggling since the API was returning insanely high value for expirationSeconds which was unexpected.

So, I started examining the code in the bottom layers, and behold I found the reason. Below was the line that was causing this problem:

public DateTime GetSessionExpiration(string SessionRefId)
{
  return _userContextService.GetUserSessionExpiration(SessionRefId) ?? DateTime.MinValue;
}

In the above code, the programmer was setting a default value of DateTime type to DateTime.MinValue which is ‘0001–01–01’ and at the time of consumption of this value at a higher level, another null check was applied, and a default value was assigned there as well.

So, I corrected it as below:

public DateTime? GetSessionExpiration(string SessionRefId)
{
  return _userContextService.GetUserSessionExpiration(SessionRefId);
}

and the issue was gone!!

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