shaders not compiling properly in monodevelop?

hi, im having real trouble with some simple custom shaders - using the latest patch of 2017 and monodevelop - i paste the code into a shader file but when i hot compile in monodevelop it doesnt show up in the shader drop down list, unless i go back to monodevelop and save the code using 'save as', so then it appears, but i get an error that this shader and fallbacks are not supported on this gpu - ive got an nvidia gtx 1050, so i cant imagine its not supported, plus its a very simple shader... also it appears its not compiling properly because in the script there is code for main_tex, but in the shaderthere is no space to put a texture?

any help please?

paul uk

 Shader "Cg  shader for billboards" {
    Properties {
       _MainTex ("Texture Image", 2D) = "white" {}
       _ScaleX ("Scale X", Float) = 1.0
       _ScaleY ("Scale Y", Float) = 1.0
    }
    SubShader {
       Pass {   
          CGPROGRAM
  
          #pragma vertex vert  
          #pragma fragment frag
 
          // User-specified uniforms            
          uniform sampler2D _MainTex;        
          uniform float _ScaleX;
          uniform float _ScaleY;
 
          struct vertexInput {
             float4 vertex : POSITION;
             float4 tex : TEXCOORD0;
          };
          struct vertexOutput {
             float4 pos : SV_POSITION;
             float4 tex : TEXCOORD0;
          };
  
          vertexOutput vert(vertexInput input) 
          {
             vertexOutput output;
 
             output.pos = mul(UNITY_MATRIX_P, 
               mul(UNITY_MATRIX_MV, float4(0.0, 0.0, 0.0, 1.0))
               + float4(input.vertex.x, input.vertex.y, 0.0, 0.0)
               * float4(_ScaleX, _ScaleY, 1.0, 1.0));
  
             output.tex = input.tex;
 
             return output;
          }
  
          float4 frag(vertexOutput input) : COLOR
          {
             return tex2D(_MainTex, float2(input.tex.xy));   
          }
  
          ENDCG
       }
    }
 }