Vertex shader with extrusion, what am I doing wrong?

Question

So for a simple surface shader, you can use vertex modifier:

    void vert (inout appdata_full v) {
        v.vertex.xyz += v.normal * _Amount;
    }

I was thinking the same would work as vertex shader:

    // vertex shader
    VectexOutput vert (VectexInput i) {
        // init output
        VectexOutput o;
        UNITY_INITIALIZE_OUTPUT(VectexOutput, o);

        // vertex extrusion
        i.vertex.xyz += i.normal * _Multiplier;

        // world space normal
        half3 normal = UnityObjectToWorldNormal(i.normal);

        // standard diffuse lighting
        half nl = max(0, dot(normal, _WorldSpaceLightPos0.xyz));

        // input to output
        o.vertex = UnityObjectToClipPos(i.vertex);
        o.color = i.color;
        o.diff = nl * _LightColor0;
        o.diff.rgb += ShadeSH9(half4(normal, 1));
        o.uv = TRANSFORM_TEX(i.uv, _MainTex);

        return o;
    }

But it doesn't, and since surface shader is compiled to vertex/fragment shader, I am wondering what am I missing?

Answer 1

The answer is simple:

I declared _Multiplier incorrectly in the SubShader block.

It should have been uniform float _Multiplier, but I wrote uniform float4 _Multiplier;

Interesting part is: no error will be produced, because float4 _Multiplier defaults to (0,0,0,0), so i.vertex.xyz += i.normal * _Multiplier; result in zero float3, hence no change in vertex position.