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Stryker.NET wants to be a blazing fast mutation test framework. To achieve this some research has taken place before designing the framework.


The framework had to conform the following criteria:

  • Be fast
  • Be able to show the exact location of the mutations to the user
  • The following mutations should be able to be made
    • Binary mutations
    • Boolean mutations
    • Logical mutations
    • Conditional mutations
    • Unary mutations
    • Return value mutations


By looking at other frameworks two options could be found regarding architectures:

Later another architecture was found: mutant schemata (also dubbed mutation switching). This technique places all mutations inside if statements. Such an if statement could look like:

if(Environment.GetEnvironmentVariable("ActiveMutation") == "1") {
i--; // mutated code
} else {
i++; // original code


For each option a separate prototype has been created. The results showed the following pros and cons.

Mutating source code​


  • Exact location can be shown to users.


  • Each mutation has to be compiled separate. So mutating is slow.

Mutating byte code​


  • Fast


  • Mutators are difficult to create.
  • Exact location cannot be shown to users.

Mutant schemata (mutation switching)​


  • All mutants can be compiled at once, so mutating is fast.
  • Exact location can be shown to users.
  • Mutated assembly can be kept in memory during mutation testruns.
  • Mutation coverage can be easily calculated.
  • Testing multiple mutations in one testrun is possible.


  • Not all mutations are possible
    • Mutating constant values
    • Mutating method names
    • Mutating access modifiers
  • Compile errors should not occur.
    • Since all mutations will compile at once, all mutations should compile correctly


Mutant schemata works fastest and the exact location for every mutation can be shown to the user. Stryker.NET has chosen the path of mutant schemata.