Read Only Attribute for Unity (just mark stuff as [ReadOnly] the same way you would use [HideInInspector])

ReadOnlyAttribute.cs

using UnityEngine;
using System;
using System.Reflection;
using System.Text.RegularExpressions;

[AttributeUsage (AttributeTargets.Field,Inherited = true)]
public class ReadOnlyAttribute : PropertyAttribute {}

#if UNITY_EDITOR
[UnityEditor.CustomPropertyDrawer (typeof(ReadOnlyAttribute))]
public class ReadOnlyAttributeDrawer : UnityEditor.PropertyDrawer
{
	public override void OnGUI(Rect rect, UnityEditor.SerializedProperty prop, GUIContent label)
	{
		bool wasEnabled = GUI.enabled;
		GUI.enabled = false;
		UnityEditor.EditorGUI.PropertyField(rect,prop);
		GUI.enabled = wasEnabled;
	}
}
#endif

Unfortunately it doesn't fully work for Lists:

[ReadOnly] public List<int> test = new List<int>(){ 1,2,3};

The most "dangerous" value to change is till available.

And it especially doesn't work for nested elements:

where the elements are of type

[Serializable]
public class TestClass
{
    public int num;
    public string text;
}

[ReadOnly] public List<TestClass> test = new List<TestClass>(){
    new TestClass(),
    new TestClass{num = 2, text = "Hi"},
    new TestClass{num = 3, text = "asd"}
};

also if it is not in a list like

[ReadOnly] public TestClass test = new TestClass();

A few years later but I solved this issue by replacing

• UnityEditor.EditorGUI.PropertyField(rect,prop);
  with:
• EditorGUILayout.PropertyField(property, label, true);