For projections of the form Select(p => new SomeType { MemberA = <expr> }), MongoProjectionBindingExpressionVisitor.VisitMemberAssignment pushes the binding member name (MemberA) onto the projection-member stack. When ApplyProjection flattens the mapping, that name becomes the alias of the projection entry — even when the underlying expression has nothing to do with a property called MemberA.

  using var db = GuidesDbContext.Create(database.MongoDatabase);
  // Planet.name is a string EF property; p.orderFromSun is an int.
  // Alias becomes "name" (the binding member), the shaper resolves Planet.FindProperty("name"),
  // reads its nullability (true), and applies it to the int read.
  var results = db.Planets
      .Select(p => new Planet { name = p.orderFromSun.ToString() })
      .ToList(); 

Which throws System.ArgumentException : Method 'System.String ToString()' declared on type 'System.Int32' cannot be called with instance of type 'System.Nullable`1[System.Int32]'