GeometryShader Object
A geometryshader object processes entire primitives. Use the following syntax to declare a geometryshader object.
[maxvertexcount(NumVerts)] void ShaderName ( PrimitiveType DataType Name [ NumElements ], inout StreamOutputObject ); 
Parameters

[maxvertexcount(NumVerts)]

[in] Declaration for the maximum number of vertices to create.
 [maxvertexcount()]  required keyword; brackets and parenthesis are required characters for correct syntax.
 NumVerts  An integer number representing the number of vertices.

ShaderName

[in] An ASCII string that contains a unique name for the geometryshader function.

PrimitiveType DataType Name [ NumElements ]

PrimitiveType  Primitive type, which determines the order of the primitive data.
Primitive Type Description point Point list line Line list or line strip triangle Triangle list or triangle strip lineadj Line list with adjacency or line strip with adjacency triangleadj Triangle list with adjacency or triangle strip with adjacency DataType  [in] An input data type; can be any HLSL data type.
Name  Argument name; this is an ASCII string.
NumElements  Array size of the input, which depends on the PrimitiveType as shown in the following table.
Primitive Type NumElements point [1]
You operate on only one point at a time.line [2]
A line requires two vertices.triangle [3]
A triangle requires three vertices.lineadj [4]
A lineadj has two ends; therefore, it requires four vertices.triangleadj [6]
A triangleadj borders three more triangles; therefore, it requires six vertices. 
StreamOutputObject

The declaration of the streamoutput object.
Return Value
None
Remarks
The following diagram shows the various primitive types for a geometry shader object.
The following diagram shows geometry shader invocations.
Examples
This example is from exercise 1 from the Direct3D 10 Shader Model 4.0 Workshop.
[maxvertexcount(3)]
void GSScene( triangleadj GSSceneIn input[6], inout TriangleStream<PSSceneIn> OutputStream )
{
PSSceneIn output = (PSSceneIn)0;
for( uint i=0; i<6; i+=2 )
{
output.Pos = input[i].Pos;
output.Norm = input[i].Norm;
output.Tex = input[i].Tex;
OutputStream.Append( output );
}
OutputStream.RestartStrip();
}
Minimum Shader Model
This object is supported in the following shader models.
Shader Model  Supported 

Shader Model 4 and higher shader models  yes 