st.cs

// switch on type?
using System;

class App {
	public static void Main ()
	{
		sbyte v;
		if (TryConvert (42.0, out v)) {
			Console.WriteLine ($"v={v}");
		}
	}
	
	internal static bool TryConvertSByte (double value, out sbyte outValue)
	{
		if (value < sbyte.MinValue || value > sbyte.MaxValue)
		{
			outValue = 0;
			return false;
		}

		outValue = (sbyte) Math.Round(value);
		return true;
	}

	internal static bool TryConvert<T>(double value, out T outValue)
		where T : struct
	{
		outValue = default (T);

		switch (typeof (T)) {
		case Type sbyteType when sbyteType == typeof (sbyte):
			sbyte v;
			if (TryConvertSByte (value, out v)) {
				outValue = (T) (object) v;
				return true;
			}
			break;
		default:
			break;
		}
		return false;
	}
}