Passing multiple parameters into SQL Server stored procedure

Let's say you are passing EmployeeIDs, and they are integers. First, create a table type in the database:

CREATE TYPE dbo.EmployeesTVP AS TABLE(EmployeeID INT PRIMARY KEY);

Now your stored procedure can say:

CREATE PROCEDURE dbo.GetEmployees
  @empTVP dbo.EmployeesTVP READONLY
AS
BEGIN
  SET NOCOUNT ON;

  SELECT EmployeeID, Name FROM dbo.Employees AS e
    WHERE EXISTS (SELECT 1 FROM @empTVP WHERE EmployeeID = e.EmployeeID);
END
GO

And if you want to handle the all scenario, you can say:

IF EXISTS (SELECT 1 FROM @empTVP)

  SELECT EmployeeID, Name FROM dbo.Employees AS e
    WHERE EXISTS (SELECT 1 FROM @empTVP WHERE EmployeeID = e.EmployeeID);

ELSE

  SELECT EmployeeID, Name FROM dbo.Employees;

(You could combine these with an OR conditional, but I tend to find this just gives the optimizer fits.)

Then you create a DataTable in your C# code from the checkboxes, and pass the parameter to your stored procedure as a parameter type of Structured. You can fill in the rest but:

DataTable tvp = new DataTable();

// define / populate DataTable from checkboxes

using (connectionObject)
{
  using (SqlCommand cmd = new SqlCommand("dbo.GetEmployees", connectionObject))
  {
    cmd.CommandType = CommandType.StoredProcedure;
    SqlParameter tvparam = cmd.Parameters.Add("@empTVP", SqlDbType.Structured);
    tvparam.TypeName = "dbo.EmployeesTVP";
    tvparam.Value = tvp;
    // ... execute the cmd, grab a reader, etc.
  }
}