//to add a new ridgidbody
gameobject.AddComponent<Rigidbody>();
//to add a new meshCollider
gameobject.AddComponent<MeshCollider>();
//Original answer by MakerBenjammin6, cleanup by me.
//Use the AddComponent<T>() Method to add a component to your gameobject
GameObject gameObject;
gameObject.AddComponent<BoxCollider>(); //Component has to be an actual Unity component
// Consider an existing GameObject, gameObj
Rigidbody addedComp = gameObj.AddComponent<Rigidbody>() as Rigidbody;
// You can also add custom components by class name
//This uses C# and unity
//to add a ridgidbody
gameobject.addComponet<ridgidbody>();
//to add a meshCollider
gameobject.addComponet<MeshCollider>();
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