using E = System.Linq.Expressions.Expression;
...
void theAlgorithm<T>(T someVar, string field, string creationFunction) where T : BaseSomeType{
    E.Lambda<Action>(
        E.Assign(
            E.Field(E.Constant(someVar), field),
            E.Call(E.Constant(this), creationFunction, null)
	)
    ).Compile()();
 
    lock (something){
        T someOtherVar = (T)something.Find(someVar.unrelatedMember);
 
        if (someOtherVar != null){
            E.Lambda<Action>(
                E.Assign(
                    E.Field(E.Constant(someOtherVar), field),
                    E.Field(E.Constant(someVar), field)
                )
            ).Compile()();
        }
    }
}

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