Property Value

null if this code element is defined in a third-party assembly.
null if the PDB file of this assembly or code element parent's assembly, hasen't been found.
null if this code element is an interface, an enumeration, an abstract method or a default constructor.
Otherwise returns the number of logical lines of code for this code element.

This metric (known as LoC) can be computed only if PDB files are present.
NDepend computes this metric directly from the information provided in PDB files.
The LoC for a method is equal to the number of sequence points found for this method in the PDB file.
A sequence point is used to mark a spot in the IL code that corresponds to a specific location in the original source.
More info about sequence points can be found here
Notice that sequence points which correspond to C# braces‘{‘ and ‘}’ are not taken account.
Computing the number of lines of code from PDB’s sequence points allows to obtain a logical LoC of code instead of a physical LoC (i.e directly computed from source files).
Two significant advantages of logical LoC over physical LoC are:

• Coding style doesn’t interfere with logical LoC. For example the LoC won’t change because a method call is spawn on several lines because of a high number of arguments.
• Logical LoC is independent from the programming language. Values obtained from assemblies written with different languages are comparable and can be summed.

Notice that the LoC for a type is the sum of its methods’ LoC, the LoC for a namespace is the sum of its types’ LoC, the LoC for an assembly is the sum of its namespaces’ LoC and the LoC for a code base is the sum of its application assemblies LoC.
Here are some observations:

• Interfaces, abstract methods and enumerations have a NbLinesOfCode value equals to null. Only concrete code that is effectively executed is considered when computing LoC.
• Namespaces, types, fields and methods declarations are not considered as line of code because they don’t have corresponding sequence points.
• When the C# or VB.NET compiler faces an inline instance fields initialization, it generates a sequence point for each of the instance constructor (the same remark applies for inline static fields initialization and static constructor).
• LoC computed from an anonymous method doesn’t interfere with the LoC of its outer declaring methods.
• The overall ratio between NbILInstructions and LoC (in C# and VB.NET) is usually around 7.

Recommendations:
Methods where NbLinesOfCode is higher than 20 are hard to understand and maintain.
Methods where NbLinesOfCode is higher than 40 are extremely complex and should be split in smaller methods (except if they are automatically generated by a tool).

Reference

ICodeContainer Interface
ICodeContainer Members