求教Linq 动态生成 dblinq或者lambda 或者构建Expressions

C# code

using (LongXingDBDataContext db = new LongXingDBDataContext()){                  this.dataGridView1.DataSource = from d in db.ZKJCXX                                   where d.ZKID.Contains(this.tbWellPointID.Text.Trim())                                   && d.ZM.Contains(this.tbWellPointName.Text.Trim())                                   && !d.BF                                   //需要判断combox的值来决定是否添加一条子查询                                   //if(this.cmbUnit.SelectedValue == "-1")                                   //{                                    && d.ZB.Equals(this.cmbWellType.SelectedValue)                                    //}                                                        &&d.SSDW.Equals(this.cmbUnit.SelectedValue)                                                    select new { 站库ID = d.ZKID, 站库名 = d.ZM, 站别名称 = [/align]d.ZBMC, 所属单位 = d.SSDWMC };                }

问题,需要根据外部条件来动态生成lambd表达式或者dblinq但是无从下手...
有高人指点用linq.expressions但是看msdn的代码太少请帮忙解决
注:不要拼接sql字符串的方法

[解决办法]

C# code

using (LongXingDBDataContext db = new LongXingDBDataContext()){    var obj = db.ZKJCXX.Where(p=>p.ZKID.Contains(this.tbWellPointID.Text.Trim()))                       .Where(p=>p.ZM.Contains(this.tbWellPointName.Text.Trim())                       .Where(p=>!p.BF)                       .Where(p=>p.SSDW.Equals(this.cmbUnit.SelectedValue));    if(this.cmbUnit.SelectedValue == "-1")    {             obj = obj.Where(p=>p.ZB.Equals(this.cmbWellType.SelectedValue));     }      this.dataGridView1.DataSource = from d in obj                                         select new { 站库ID = d.ZKID, 站库名 = d.ZM, 站别名称 = [/align]d.ZBMC, 所属单位 = d.SSDWMC };     }