In MontiCore, the TypeCheck is used to calculate the type of a set of expressions. This happens by traversing the AST of an expression, calculating the types of its sub-expressions and combining them to the type of the main expression.

Given Classes in MontiCore

I want to write a CoCo for my language that uses the TypeCheck - How?

You can use the TypeCheck facade. The facade needs a Derive-Class (for expressions and literals) and/or a Synthesize-Class (for types) and calculates the type of your given expressions/literals/types.

Create a DelegatorVisitor which combines all expression grammars and literal grammars used by your language. The DelegatorVisitor has to implement the Interface ITypesCalculator. Use this Delegator as Derive-Class in the TypeCheck facade. The Synthesize-Class depends on the types grammar you use (see above-mentioned classes). For an example of the Delegator-Visitor see here.

If you want to create a Derive-Class for your expression/literal grammar, you have to extend the Derive-Class of the supergrammar and implement the normal visitor of your language. There you can override the traverse methods for your expressions/literals so that it calculates the type of the expression/literal (see implementation in one of the above-mentioned classes). You can add your visitor to the DelegatorVisitor later on. For an example of a Derive-Class see here.

Writing a CoCo to check the correctness of your type/expression/literal should be easy now that you have the TypeCheck facade to use. Just use the correct Derive-Class and/or the correct Synthesize-Class as parameters and check if the type of your expression or type is correctly calculated.

Example for a CoCo:

public void check(ASTExpression expr){
    YourDeriveClass deriveClass = new YourDeriveClass(...); //instance of your Derive-Class
    YourSynthesizeClass synthesizeClass = new YourSynthesizeClass(...); //instance of your Synthesize-Class
    TypeCheck check = new TypeCheck(synthesizeClass,deriveClass); //instance of the TypeCheck-facade, parameters are your Synthesize-Class and your Derive-Class
    if("double"!=check.typeOf(expr).print()){ //test if your expression is of the correct type (here: double)
        Log.error(...); //your specified error message