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System.NullReferenceException

"Object reference not set to an instance of an object"

While there is no need to instantiate the basic local variables, like int, bool, double, etc. most other objects need to be created by calling the constructor with the magic word "new". If methods or properties of the object are accessed without the object being instantiated, a null reference exception will be thrown.

Three special cases:

  • String variables can be null (as it is a reference type), but do not have the "new" key word to be instantiated. Simply assign a value to them.
  • Static methods and properties can be accessed without calling the constructor.
  • A nullable int, bool, double, etc. can be defined by adding a question mark, e.g. double? bitrate;

Example 1

Data enters in JSON format, which is being deserialized. Suppose that you want to know if the street property that is present in person.LocationInfo.Street starts with "Test".

If you were sure that all the items are in there, then the simple call would be this:

if (person.LocationInfo.Street.StartsWith("Test"))

However, the problem is that person, LocationInfo, or Street could be null, and accessing items of these members could generate null reference exceptions.

Solution 1.A

Verify every member before accessing it:

if(person != null 
&& person.LocationInfo != null 
&& person.LocationInfo.Street != null 
&& person.LocationInfo.Street.StartsWith("Test"))

The downside of this is that when the depth of items to check is too large, SonarQube will complain that there are too many conditional operators used in the expression. It will also affect the complexity calculation of the code.

Solution 1.B

Since C# 6 and later, a null-conditional member access operator ?. is available, also known as the Elvis operator. This will return null when one of the members is null. This can then be combined with the null-coalescing operator ??.

if (person?.LocationInfo?.Street?.StartsWith("Test") ?? false)

If person, LocationInfo or Street is empty, null will be returned. With the ?? check, this will be translated to false, so that the content inside the if clause will not be executed. If none of the members are null, the normal result of the StartsWith method will be used.

Example 2

Converting to a string can give different results:

string folder = Convert.ToString(protocol.GetParameter(Parameter.data_offload_folder_5000));
if (folder.EndsWith("\\"))

Even though the protocol.GetParameter method will return null when the parameter is not initialized, using Convert.ToString on a null object will translate this into a valid referenced empty string on which you can safely call the members or methods such as ".EndsWith".

Suppose that string folder = Convert.ToString(person.Folder); and person.Folder is a string type but also null, then Convert.ToString on a null string will return a null string. When methods such as ".EndsWith" are called, this will result in a null reference exception.

Solution 2.A

Add an extra check to make sure the string is not null or empty before executing the method:

if (!String.IsNullOrEmpty(folder) && folder.EndsWith("\\"))

Solution 2.B

Use the Elvis operator (requires C# 6):

if (folder?.EndsWith("\\") ?? false)
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