Genericized #Unity3D SerializedProperty value access, an extension that should be part of the API.

SerializedPropertyValueExtension.cs

/// @creator: Slipp Douglas Thompson
/// @license: Public Domain per The Unlicense.  See <http://unlicense.org/>.
/// @purpose: Genericized Unity3D SerializedProperty value access.
/// @why: Because this functionality should be built-into Unity.
/// @usage: Use as you would a native SerializedProperty method;
/// 	e.g. `Debug.Log(mySerializedProperty.Value<Color>());`
/// @intended project path: Assets/Plugins/Editor/UnityEditor Extensions/SerializedPropertyValueExtension.cs
/// @interwebsouce: https://gist.github.com/capnslipp/8516384

using System;
using System.Reflection;
using System.Collections.Generic; // for: KeyValuePair<,>
using UnityEngine;
using UnityEditor;



public static class SerializedPropertyValueExtension
{
	/// @note: switch/case derived from the decompilation of SerializedProperty's internal SetToValueOfTarget() method.
	public static ValueT Value<ValueT>(this SerializedProperty thisSP)
	{
		Type valueType = typeof(ValueT);
		
		// First, do special Type checks
		if (valueType.IsEnum)
			return (ValueT)Enum.ToObject(valueType, thisSP.enumValueIndex);
		
		// Next, check for literal UnityEngine struct-types
		// @note: ->object->ValueT double-casts because C# is too dumb to realize that that the ValueT in each situation is the exact type needed.
		// 	e.g. `return thisSP.colorValue` spits _error CS0029: Cannot implicitly convert type `UnityEngine.Color' to `ValueT'_
		// 	and `return (ValueT)thisSP.colorValue;` spits _error CS0030: Cannot convert type `UnityEngine.Color' to `ValueT'_
		if (typeof(Color).IsAssignableFrom(valueType))
			return (ValueT)(object)thisSP.colorValue;
		else if (typeof(LayerMask).IsAssignableFrom(valueType))
			return (ValueT)(object)thisSP.intValue;
		else if (typeof(Vector2).IsAssignableFrom(valueType))
			return (ValueT)(object)thisSP.vector2Value;
		else if (typeof(Vector3).IsAssignableFrom(valueType))
			return (ValueT)(object)thisSP.vector3Value;
		else if (typeof(Rect).IsAssignableFrom(valueType))
			return (ValueT)(object)thisSP.rectValue;
		else if (typeof(AnimationCurve).IsAssignableFrom(valueType))
			return (ValueT)(object)thisSP.animationCurveValue;
		else if (typeof(Bounds).IsAssignableFrom(valueType))
			return (ValueT)(object)thisSP.boundsValue;
		else if (typeof(Gradient).IsAssignableFrom(valueType))
			return (ValueT)(object)SafeGradientValue(thisSP);
		else if (typeof(Quaternion).IsAssignableFrom(valueType))
			return (ValueT)(object)thisSP.quaternionValue;
		
		// Next, check if derived from UnityEngine.Object base class
		if (typeof(UnityEngine.Object).IsAssignableFrom(valueType))
			return (ValueT)(object)thisSP.objectReferenceValue;
		
		// Finally, check for native type-families
		if (typeof(int).IsAssignableFrom(valueType))
			return (ValueT)(object)thisSP.intValue;
		else if (typeof(bool).IsAssignableFrom(valueType))
			return (ValueT)(object)thisSP.boolValue;
		else if (typeof(float).IsAssignableFrom(valueType))
			return (ValueT)(object)thisSP.floatValue;
		else if (typeof(string).IsAssignableFrom(valueType))
			return (ValueT)(object)thisSP.stringValue;
		else if (typeof(char).IsAssignableFrom(valueType))
			return (ValueT)(object)thisSP.intValue;
		
		// And if all fails, throw an exception.
		throw new NotImplementedException("Unimplemented propertyType "+thisSP.propertyType+".");
	}
	
	public static dynamic Value(this SerializedProperty thisSP)
	{
		switch (thisSP.propertyType) {
			case SerializedPropertyType.Integer:
				return thisSP.intValue;
			case SerializedPropertyType.Boolean:
				return thisSP.boolValue;
			case SerializedPropertyType.Float:
				return thisSP.floatValue;
			case SerializedPropertyType.String:
				return thisSP.stringValue;
			case SerializedPropertyType.Color:
				return thisSP.colorValue;
			case SerializedPropertyType.ObjectReference:
				return thisSP.objectReferenceValue;
			case SerializedPropertyType.LayerMask:
				return thisSP.intValue;
			case SerializedPropertyType.Enum:
				int enumI = thisSP.enumValueIndex;
				return new KeyValuePair<int, string>(enumI, thisSP.enumNames[enumI]);
			case SerializedPropertyType.Vector2:
				return thisSP.vector2Value;
			case SerializedPropertyType.Vector3:
				return thisSP.vector3Value;
			case SerializedPropertyType.Rect:
				return thisSP.rectValue;
			case SerializedPropertyType.ArraySize:
				return thisSP.intValue;
			case SerializedPropertyType.Character:
				return (char)thisSP.intValue;
			case SerializedPropertyType.AnimationCurve:
				return thisSP.animationCurveValue;
			case SerializedPropertyType.Bounds:
				return thisSP.boundsValue;
			case SerializedPropertyType.Gradient:
				return SafeGradientValue(thisSP);
			case SerializedPropertyType.Quaternion:
				return thisSP.quaternionValue;
			
			default:
				throw new NotImplementedException("Unimplemented propertyType "+thisSP.propertyType+".");
		}
	}
	
	/// Access to SerializedProperty's internal gradientValue property getter, in a manner that'll only soft break (returning null) if the property changes or disappears in future Unity revs.
	static Gradient SafeGradientValue(SerializedProperty sp)
	{
		BindingFlags instanceAnyPrivacyBindingFlags = BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance;
		PropertyInfo propertyInfo = typeof(SerializedProperty).GetProperty(
			"gradientValue",
			instanceAnyPrivacyBindingFlags,
			null,
			typeof(Gradient),
			new Type[0],
			null
		);
		if (propertyInfo == null)
			return null;
		
		Gradient gradientValue = propertyInfo.GetValue(sp, null) as Gradient;
		return gradientValue;
	}
}

Usefull but for some reason when I try to use this for a LayerMask it gives the next error:

"InvalidCastException: Cannot cast from source type to destination type.
CustomPlugins.SerializedPropertyValueExtension.Value[LayerMask](UnityEditor.SerializedProperty thisSP) (at Assets/Plugins/SerializedPropertyValueExtension.cs:28)
PlayerCustomEditor.OnInspectorGUI () (at Assets/Editor/Player/PlayerCustomEditor.cs:35)
UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor editor, Int32 editorIndex, Boolean forceDirty, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect, Boolean eyeDropperDirty) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1162)
UnityEditor.DockArea:OnGUI()"

I testing with:

SerializedObject myScript = new SerializedObject(target);
SerializedProperty WhatIsGround = myScript.FindProperty("WhatIsGround");

Debug.Log(WhatIsGround.Value< LayerMask >());

public static dynamic Value(this SerializedProperty thisSP)

dynamic generates an error "Cannot define a class or member that utilizes 'dynamic' because the compiler required type 'System.Runtime.CompilerServices.DynamicAttribute' cannot be found. Are you missing a reference?"

How do I fix this? I'm using VisualStudio 2015 Enterprise Edition with Unity integration.

As long as I know the version of c# in Unity does not support dynamic which is available over c# 4.0