Why doesn't BCL have methods like DateTime.WriteTo(TextWriter) etc.

[TahirAhmadov]

When calling stringBuilder.Append(dt); or textWriter.Write(dt);, it boxes to object, and does a virtual call on ToString - this is 2 unnecessary allocations. Those classes do have overloads for int and some other primitive types, but the set of overloads is limited, and it's not a scalable or maintainable approach.

It would be much more efficient to have:

interface IAppendString
{
  void AppendTo(StringBuilder sb); // whatever parameters/overloads are needed here for format, culture etc.
}
struct DateTime : ..., IAppendString
{
  public void AppendTo(StringBuilder sb)
  {
    // ...
  }
}
class StringBuilder // similar for TextWriter
{
  public void Append<T>(T value) where T : IAppendString
  {
    value.AppendTo(this);
  }
}
var sb = new StringBuilder();
sb.Append(DateTime.Now); // no allocations (other than potential char[] manipulations inside StringBuilder itself)

This approach can be applied to all types - primitives like int, useful BCL types like DateTime, and all other non-BCL types in the eco-system.

[huoyaoyuan]

Why doesn't BCL have methods like DateTime.WriteTo(TextWriter) etc.

This is against layering. Data types are core while TextWriter is at a higher layer. Efficiently writing to TextWriter isn't special than writing to other things: just format the object into a buffer of UTF-8 or UTF-16, then copy the buffer to any writer. Or, just format to the destination buffer directly.

This lead to the current solution: interpolated string handler with IFormattable/IUtf8Formattable. StringBuilder already implements this scheme.