Vertex definitions and shaders

I noticed that from looking at other examples like say .. riemers tutorials he takes a buffer with a bunch of vector3′s in it and ties it to a shader which expects a float4 … why does this work in his situation and not mine?

Also is there a simple fix for this situation that will allow me to do this with the shader determining the w component as to my game logic this means nothing but is obviously crucial to the gpu.

Riemers code is here:

http://www.riemers.net/eng/Tutorials/XNA/Csharp/Series4/Textured_terrain.php

and mine (key parts only) …

CPU Code:

public struct TexturedVertex: IVertex
{
    public Vector3 Position { get; set; }
    public Vector2 Uv { get; set; }

    public TexturedVertex(Vector3 position, Vector2 uv) : this()
    {
        Position = position;
        Uv = uv;
    }
}

Shader Code:

struct VS_IN
{
    float4 pos : POSITION;
    float2 tex : TEXCOORD;
};

struct PS_IN
{
    float4 pos : SV_POSITION;
    float2 tex : TEXCOORD;
};

Texture2D picture;
SamplerState pictureSampler;

PS_IN VS(float4 inPos : POSITION, float2 uv : TEXCOORD)
{
    PS_IN output = (PS_IN)0;
    output.pos = mul(inPos, mul(World, ViewProjection));
    output.tex = uv;
    return output;
}

How do the two tie together?

I am however using sharpDX not XNA so my code for setting up the buffers is different slightly …

I created my own mesh class that does this:

VertexBuffer = Buffer.Create(device, BindFlags.VertexBuffer ,Vertices.ToArray());
context.InputAssembler.SetVertexBuffers(0, new VertexBufferBinding(VertexBuffer, Utilities.SizeOf<TexturedVertex>(), 0));