private Expression<Func<Blog, bool>> getCondition()
{
Expression<Func<Blog, bool>> expression = blog => true;
if (!String.IsNullOrEmpty(Request["BlogClassID"]))
{
int blogClassID;
if (Int32.TryParse(Request["BlogClassID"], out blogClassID))
{
Expression<Func<Blog, bool>> e2 = blog => blog.BlogClass == null;
var invokedExpr = Expression.Invoke(e, expression.Parameters.Cast<Expression>());
expression = Expression.Lambda<Func<Blog, bool>>(Expression.And(expression.Body, invokedExpr), expression.Parameters);
}
}
return expression;
}

var result = new DongBlogDataContext().Blogs.Where(getCondition());

SELECT [t0].[BlogID], [t0].[ChannelID], [t0].[BlogClassID], [t0].[Title], [t0].[Content], [t0].[Tag], [t0].[CreateDateTime]
FROM [dbo].[Blog] AS [t0]
WHERE [t0].[BlogClassID] IS NULL

Expression<Func<Blog, bool>> e = blog => blog.BlogClass == null;
var invokedExpr = Expression.Invoke(e, expression.Parameters.Cast<Expression>());
expression = Expression.Lambda<Func<Blog, bool>>(Expression.And(expression.Body, invokedExpr), expression.Parameters);