Table of Contents

TRLS017: Don't compare Result or Maybe to null

Cause

Comparing a Result<T> or Maybe<T> value to null using ==, !=, is null, or is not null.

Rule Description

Result<T> and Maybe<T> are structs (value types), not classes. They cannot be null. Comparing them to null:

  • Always produces a compile error (CS0019 or CS9135)
  • Indicates a misunderstanding of how these types work
  • Should use proper state checks instead

How to Fix Violations

Use the appropriate state-checking properties:

For Result

// ❌ Bad - Result is a struct, can't be null
if (result == null) { }
if (result is null) { }
if (result != null) { }
if (result is not null) { }

// ✅ Good - Use state properties
if (result.IsSuccess) { }
if (result.IsFailure) { }

For Maybe

// ❌ Bad - Maybe is a struct, can't be null
if (maybe == null) { }
if (maybe is null) { }

// ✅ Good - Use state properties
if (maybe.HasValue) { }
if (maybe.HasNoValue) { }

Examples

Example 1: Checking Result Success

// ❌ Bad - Compiler error + wrong pattern
public IActionResult Get(Guid id)
{
    var result = GetCustomer(id);
    if (result == null)  // CS0019: Operator '==' cannot be applied
        return NotFound();
    return Ok(result.Value);
}

// ✅ Good
public IActionResult Get(Guid id)
{
    var result = GetCustomer(id);
    if (result.IsFailure)
        return result.Error.ToHttpResult();
    return Ok(result.Value);
}

Example 2: Checking Maybe Value

// ❌ Bad
public string GetDisplayName(Maybe<User> maybeUser)
{
    if (maybeUser is null)  // Compiler error
        return "Guest";
    return maybeUser.Value.Name;
}

// ✅ Good
public string GetDisplayName(Maybe<User> maybeUser)
{
    if (maybeUser.HasNoValue)
        return "Guest";
    return maybeUser.Value.Name;
}

// ✅ Better - Use Match
public string GetDisplayName(Maybe<User> maybeUser) =>
    maybeUser.Match(
        onValue: user => user.Name,
        onNoValue: () => "Guest");

Example 3: Pattern Matching

// ❌ Bad - Wrong pattern
var message = result is not null ? "Has value" : "No value";

// ✅ Good
var message = result.IsSuccess ? "Success" : "Failure";

// ✅ Better - Use Match
var message = result.Match(
    onSuccess: _ => "Success",
    onFailure: _ => "Failure");

Why Structs Can't Be Null

In C#, structs are value types that:

  • Are stored on the stack (or inline in other objects)
  • Always have a value (default is default(T), not null)
  • Cannot be assigned null without using Nullable<T> (T?)

Result<T> and Maybe<T> are intentionally designed as structs to:

  • Avoid null reference exceptions
  • Provide better performance (no heap allocation)
  • Force explicit handling of success/failure states
  • TRLS003 - Unsafe access to Result.Value
  • TRLS006 - Unsafe access to Maybe.Value

See Also

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