In TypeScript, you can create a type called C with the fields A and B (both strings), and C (a boolean) using the following syntax:

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type C = {
  A: string;
  B?: string; // the '?' indicates that this field is optional
  C: boolean;
}

To specify that B is required if C is true, you can use an intersection type and a type guard:

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type C = {
  A: string;
  C: boolean;
} & (
  { // this type is only valid if C is true
    B: string;
  }
);

function isCWithB(c: C): c is C {
  return c.C && c.B !== undefined;
}

Then, you can use the isCWithB function as a type guard to narrow the type of the C object and access the B field safely:

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const c: C = {
  A: 'foo',
  C: true,
  B: 'bar'
};

if (isCWithB(c)) {
  // here, the type of c is narrowed to {A: string; C: boolean; B: string}
  console.log(c.B); // outputs 'bar'
}

Keep in mind that this solution only works if the B field is optional in the C type, otherwise TypeScript will raise an error when you try to use the B field in the type guard.