Fog not working in my Shader~

in Unity5 , Fog not working in my own Vertext Shader,i use default surface shader is working.

I see the documentation:

 Fog handling was changed
      
      Unity 5.0 makes built-in Fog work on WP8 and consoles, but in order to achieve that we’ve changed how Fog is done a bit. For surface shaders and fixed function shaders, nothing extra needs to be done - fog will be added automatically (you can add “nofog” to surface shader #pragma line to explicitly make it not support fog).
      
      For manually written vertex/fragment shaders, fog does not happen automagically now. You need to add
      #pragma multi_compile_fog and fog handling macros to your shader code. Check out built-in shader source, for example Unlit-Normal how to do it.

i add the #pragma multi_compile_fog , but still not working...

there is my code:

   Properties {
          _MainTex ("Base (RGB)", 2D) = "white" {}
          _QOffset ("Offset", Vector) = (0,0,0,0)
          _Dist ("Distance", Float) = 100.0
      }
    
  SubShader {
      Tags { "RenderType"="Opaque"  }
      Fog  {Mode Linear }
      Pass
      {
          CGPROGRAM
          #pragma vertex vert
          #pragma fragment frag
          #include "UnityCG.cginc"
          #pragma multi_compile_fog
           
   
          sampler2D _MainTex;
          float4 _QOffset;
          float _Dist;
   
          struct v2f {
              float4 pos : SV_POSITION;
              float4 uv : TEXCOORD0;
          };
   
          v2f vert (appdata_base v)
          {
              v2f o;
              float4 vPos = mul (UNITY_MATRIX_MV, v.vertex);
              float zOff = vPos.z/_Dist;
              vPos += _QOffset*zOff*zOff;
              o.pos = mul (UNITY_MATRIX_P, vPos);
              o.uv = mul( UNITY_MATRIX_TEXTURE0, v.texcoord );
              return o;
          }
   
          half4 frag (v2f i) : COLOR
          {
              half4 col = tex2D(_MainTex, i.uv.xy);
              return col;
          }
   
          ENDCG
      }
  }