ColorAttribute.cs

using Sandbox;
using System;
using System.Text;

namespace Hammer
{
	/// <summary>
	/// Change the color of the wireframe box in the Hammer 2D views.
	/// </summary>
	[AttributeUsage( AttributeTargets.Class )]
	public class ColorAttribute : MetaDataAttribute
	{
		internal byte ColorR;
		internal byte ColorG;
		internal byte ColorB;

		public ColorAttribute( byte r, byte g, byte b )
		{
			ColorR = r;
			ColorG = g;
			ColorB = b;
		}

		public override void AddHeader( StringBuilder sb )
		{
			sb.Append( $" color( {ColorR} {ColorG} {ColorB} )" );
		}
	}
}

DrawAnglesAttribute.cs

using Sandbox;
using System;
using System.Text;

namespace Hammer
{
	/// <summary>
	/// Draw the entity's transform angle or specific angles property in Hammer.
	/// </summary>
	[AttributeUsage( AttributeTargets.Class )]
	public class DrawAnglesAttribute : MetaDataAttribute
	{
		internal string AngleProperty;
		internal string LocalProperty;

		public DrawAnglesAttribute() { }

		public DrawAnglesAttribute( string property )
		{
			AngleProperty = property;
		}

		public DrawAnglesAttribute( string property, string local )
		{
			AngleProperty = property;
			LocalProperty = local;
		}

		public override void AddHeader( StringBuilder sb )
		{
			// drawangles()
			// drawangles(angle)
			// drawangles(angle, local)

			sb.Append( " drawangles( " );
			
			if ( !string.IsNullOrEmpty( AngleProperty ) )
			{
				sb.Append( $"{StringX.QuoteSafe( AngleProperty )}" );

				if ( !string.IsNullOrEmpty( LocalProperty ) )
					sb.Append( $", {StringX.QuoteSafe( LocalProperty )}" );
			}

			sb.Append( " ) " );
		}
	}
}

PhysicsTypeOverrideAttribute.cs

using Sandbox;
using System;
using System.Text;

namespace Hammer
{
	/// <summary>
	/// Use this to indicate to the map builder that any meshes associated with this entity should have a different default physics type.
	/// </summary>
	/// <remarks>
	/// Adds the base class PhysicsTypeOverride_* that the map builder is hard coded to use.
	/// </remarks>
	[AttributeUsage( AttributeTargets.Class )]
	public class PhysicsTypeOverrideAttribute : MetaDataAttribute
	{
		public enum PhysicsTypeOverride
		{
			Mesh,
			SingleConvex,
			MultiConvex
		};

		internal PhysicsTypeOverride PhysTypeOverride;

		public PhysicsTypeOverrideAttribute( PhysicsTypeOverride type ) => PhysTypeOverride = type;

		public override void AddHeader( StringBuilder sb )
		{
			// Seems a bit saner then relying on Enum.ToString() in the string builder..
			var name = PhysTypeOverride switch {
				PhysicsTypeOverride.Mesh => "Mesh",
				PhysicsTypeOverride.SingleConvex => "SingleConvex",
				PhysicsTypeOverride.MultiConvex => "MultiConvex",
				_ => throw new NotSupportedException()
			};

			// todo: multiple bases
			sb.Append( $" base( PhysicsTypeOverride_{PhysTypeOverride} ) " );
		}
	}
}

RenderPropertiesAttribute.cs

using Sandbox;
using System;
using System.Text;

namespace Hammer
{
	/// <summary>
	/// Use this to add Render Properties to the entity, these are used by the map builder.
	/// </summary>
	[AttributeUsage( AttributeTargets.Class )]
	public class RenderPropertiesAttribute : MetaDataAttribute
	{
		public override void AddHeader( StringBuilder sb )
		{
			// todo: multiple bases
			sb.Append( $" base( RenderFields ) " );
		}

		public override void AddBody( StringBuilder sb )
		{
			// vrad_brush_cast_shadows is used internally by the map builder on brushes
			sb.AppendLine( "\tvrad_brush_cast_shadows( boolean )[ group=\"Render Properties\" ] : \"Shadows\" : 0 : \"Set this if this brush casts lightmap shadows.\"" );

			// equally not covered in RenderFields base but still used by the map builder
			sb.AppendLine( "\tdisableshadows( boolean )[group = \"Render Properties\"] : \"Disable shadows\" : 0" );
		}
	}
}

SphereAttribute.cs

using Sandbox;
using System;
using System.Text;

namespace Hammer
{
	/// <summary>
	/// Display a sphere of the specified property's radius in Hammer.
	/// </summary>
	[AttributeUsage( AttributeTargets.Class )]
	public class SphereAttribute : MetaDataAttribute
	{
		internal string Property;

		public SphereAttribute( string property ) => Property = property;

		public override void AddHeader( StringBuilder sb )
		{
			sb.Append( $" sphere( {StringX.QuoteSafe(Property)} )" );
		}
	}
}