C# Maintainability Standards · CS-01

A method that requires more than 10 statements is simply doing too much or has too many responsibilities. It also requires the human mind to analyze the exact statements to understand what the code is doing. Break it down in multiple small and focused methods with self-explaining names, but make sure the high-level algorithm is still clear.

Roslyn Analyzer Rule ACL1002

To make a more conscious decision on which members to make available to other classes first restrict the scope as much as possible. Then carefully decide what to expose as a public member or type.

Although a property like customer.HasNoOrders make sense, avoid using it in a negative condition like this:

bool hasOrders = !customer.HasNoOrders;

Double negatives are more difficult to grasp than simple expressions, and people tend to read over the double negative easily.

Roslyn Analyzer Rule AV1502(partial)

All DLLs should be named according to the pattern ..dll where refers to your company’s name and contains one or more dot-separated clauses. For example Dnb.Web.Controls.dll.

As an example, consider a group of classes organized under the namespace Dnb.Web.Binding exposed by a certain assembly. According to this guideline, that assembly should be called Dnb.Web.Binding.dll.

EXCEPTION: If you decide to combine classes from multiple unrelated namespaces into one assembly, consider suffix to the assembly with Core, but do not use that suffix in the namespaces. For instance, Dnb.Consulting.Core.dll.

Roslyn Analyzer Rule AV1505

Use Pascal casing for naming the file and don’t use underscores.

Roslyn Analyzer Rule SA1649

Make sure that one source file can be responsible for fully or partially contributing to one class.

EXCEPTION: Nested types can, for obvious reasons, be part of the same file.

Roslyn Analyzer Rule SA1402

When using partial types and allocating a part per file, name each file after the logical part that part plays. For example:

// In MyClass.cs
public partial class MyClass
{
    ...
}

// In MyClass.Designer.cs
public partial class MyClass
{
    ...
}

Don’t use literal values, either numeric or strings, in your code other than to define symbolic constants. For example:

public class Whatever
{
    public static readonly Color PapayaWhip = new Color(0xFFEFD5);
    public const int MaxNumberOfWheels = 18;
}

Strings intended for logging or tracing are exempt from this rule. Literals are allowed when their meaning is clear from the context, and not subject to future changes, For example:

mean = (a + b) / 2;                         // okay
WaitMilliseconds(waitTimeInSeconds * 1000); // clear enough

If the value of one constant depends on the value of another, do attempt to make this explicit in the code.

public class SomeSpecialContainer
{
    public const int MaxItems = 32;
    public const int HighWaterMark = 3 * MaxItems / 4; // at 75%
}

NOTE: An enumeration can often be used for certain types of symbolic constants.

Roslyn Analyzer Rule ACL1001

Avoid the C and Visual Basic styles where all variables have to be defined at the beginning of a block, but rather define and initialize each variable at the point where it is needed.

Don’t use confusing constructs like the one below.

var result = someField = GetSomeMethod();

Roslyn Analyzer Rule RCS1081

Avoid

var startInfo = new ProcessStartInfo("myapp.exe");
startInfo.StandardOutput = Console.Output;
startInfo.UseShellExecute = true;

Instead, use Object Initialisers.

var startInfo = new ProcessStartInfo("myapp.exe")
                {
                    StandardOutput = Console.Output,
                    UseShellExecute = true
                };

Similarly, instead of adding items to collection or dictionary individually

var countries = new List<string>();
countries.Add("Netherlands");
countries.Add("United States");

Use collection or dictionary initialisers.

var countries = new List<string>
                {
                    "Netherlands",
                    "United States"
                };

It is usually bad style to compare a bool type expression to true or false. For example:

while (condition == false)                    // wrong; bad style
while (condition != true)                     // also wrong

while (condition)                             // OK

EXCEPTION: Explicit comparisons to true or false are necessary if the value being compared is a nullable bool.

Updating the loop variable within the loop body is generally considered confusing. It is even worse that the loop variables are modified in more than one place in the loop. Consider break or continue instead, to change the loop variables.

for (int index = 0; index < 10; ++index)
{
    if (some condition)
    {
        index = 11; // Wrong! Use 'break' or 'continue' instead.
    }
}

Roslyn Analyzer Rule AV1530

A method that nests loops is more difficult to understand than one with only a single loop. In fact, in most cases having nested loops can be replaced with a much simpler LINQ query that uses the from keyword twice or more to join the data.

Roslyn Analyzer Rule AV1532

Please note that this also avoids possible confusion in statements of the form:

// The wrong way:
if (b1) if (b2) Foo(); else Bar(); // which 'if' goes with the 'else'?

// The right way:
if (b1)
{
    if (b2)
    {
        Foo();
    }
    else
    {
        Bar();
    }
}

EXCEPTION: This can be omitted in the case of an if statement that is performing a null argument check, e.g. the following is ok:

if (arg == null) throw new ArgumentNullException(nameof(arg));

Roslyn Analyzer Rule ACL1006

Add a descriptive comment if the default block is supposed to be empty. Moreover, if that block is not supposed to be reached, throw an InvalidOperationException to detect future changes that may fall through the existing cases. This ensures better code, because all paths the code can travel has been thought about.

void Foo(string answer)
{
    switch (answer)
    {
        case "no":
            Console.WriteLine("You answered with No");
            break;
        case "yes":
            Console.WriteLine("You answered with Yes");
            break;
        default:
            // Not supposed to end up here.
            throw new InvalidOperationException("Unexpected answer " + answer);
    }
}

Roslyn Analyzer Rule AV1536

One entry, one exit is a sound principle and keeps control flow readable. However, if the method is very small may actually improve readability over some central boolean flag that is updated at various points.

Express your intentions directly. For example:

// Bad practice
bool pos;
if (val > 0)
{
    pos = true;
}
else
{
    pos = false;
}

// Preferred practice
bool pos = (val > 0); // initialisation

Or this can be another alternative:

// Avoid
string result;
if (someString != null)
{
    result = someString;
}
else
{
    result = "Unavailable";
}
return result;

// Instead
return someString ?? "Unavailable";

Consider the following example:

if (member.HidesBaseClassMember && (member.NodeType != NodeType.InstanceInitializer))
{
    // do something
}

In order to understand what this expression is about, you need to analyse its exact details and all the possible outcomes. Obviously, you could add an explanatory comment on top of it, but it is much better to replace this complex expression with a clearly named method like:

if (NonConstructorMemberUsesNewKeyword(member))
{
    // do something
}

private bool NonConstructorMemberUsesNewKeyword(Member member)
{
    return member.HidesBaseClassMember &&
           member.NodeType != NodeType.InstanceInitializer;
}

You still need to understand the expression if you are modifying it, but the calling code is now much easier to grasp.

This guideline only applies to overloads that are intended for providing optional arguments. Consider for example:

public class MyString
{
    private string someText;

    public MyString(string text)
    {
        this.someText = text;
    }

    public int IndexOf(string phrase)
    {
        return IndexOf(phrase, 0, someText.Length);
    }

    public int IndexOf(string phrase, int startIndex)
    {
        return IndexOf(phrase, startIndex, someText.Length - startIndex );
    }

    public virtual int IndexOf(string phrase, int startIndex, int count)
    {
        return someText.IndexOf(phrase, startIndex, count);
    }
}

The class MyString provides three overloads for the IndexOf method, but two of them simply call the one with the most parameters.

NOTE: The same rule can apply to class constructors; implement the most complete overload and call that one from the other overloads using the this() operator.

NOTE: The parameters with the same name should appear in the same position in all overloads.

IMPORTANT: If you also want to allow derived classes to override these methods, define the most complete overload as a protected virtual method that is called by all overloads.

Roslyn Analyzer Rule AV1551

If you end up with a method with more than four parameters, use a structure or class for passing multiple arguments such as explained in the Specification design pattern. In general, the fewer the number of parameters, the easier it is to understand the method. Additionally, unit testing a method with many parameters requires many scenarios to test.

Roslyn Analyzer Rule ACL1003

They can make code less understandable and might cause people to introduce bugs. Prefer returning compound objects instead.

EXCEPTION: TryParse is a common pattern that uses an out parameter appropriately.

Roslyn Analyzer Rule AV1562

Consider the following method signature:

public Customer CreateCustomer(bool platinumLevel) {}

On first sight this signature seems perfectly fine, but when calling this method you will lose this purpose completely:

Customer customer = CreateCustomer(true);

Often, a method taking such a flag is doing more than one thing and needs to be refactored into two or more methods. An alternative solution is to replace the flag with an enumeration.

Roslyn Analyzer Rule AV1564

Never use a parameter as a convenient variable for storing temporary state. Even though the type of your temporary variable may be the same, the name usually does not reflect the purpose of the temporary variable.

Roslyn Analyzer Rule AV1568

Using the is operator and pattern matching is a more concise way of safely type-casting an object. So the first code snippet is preferred over the second.

// Good
if (obj is MyClass myClass)
{
    // Do stuff
}

// Bad
var myClass = obj as MyClass;
if (myClass != null)
{
    // Do stuff
}

Never check-in code that is commented-out, but instead use a work item tracking system to keep track of some work to be done. Nobody knows what to do when they encounter a block of commented-out code. Was it temporarily disabled for testing purposes? Was it copied as an example? Should I delete it?

When defining the numeric values for an enum you should use 100, 200, etc. to allow for additional values to be added in between in future.

IMPORTANT This is a MUST when the enum has a natural order between its members, e.g. it represents a status of some kind that the system transitions through.

For example:

public enum Status
{
    None = 0,
    Draft = 100,
    InProgress = 200,
    Complete = 300
}

EXCEPTION: If the enum is a set of flags (i.e. can be treated as a bit field, decorated with the [FlagsAttribute]) then the numeric values must be powers of two.

For example, add the below to every .csproj at or above .NET Core 3 or .NET Standard 2.1. For existing codebases, this is something to work towards rather than mandated (and therefore a SHOULD), and can be introduced on a project-by-project basis if needed.

<nullable>enable</nullable>

For example, rather than this old-style switch statement:

switch (item)
{
    case Item.None:
        return 0;
    case Item.One:
        return 1;
    default:
        return 2;
}

prefer a switch expression:

return item switch
{
    Item.None => 0,
    Item.One => 1,
    _ => 2,
};

Roslyn Analyzer Rule IDE0066

For example, rather than an old-style using statement like this:

using (var stream = new MemoryStream())
{
    // do stuff
}

prefer a using declaration:

using var stream = new MemoryStream();
// do stuff

Roslyn Analyzer Rule IDE0063