任何人都可以将这个简单的LINQ-to-sql转换为表达式树：
List<Region> lst = (from r in dc.Regions
where r.RegionID > 2 && r.RegionDescription.Contains("ern")
select r).ToList();
解决方法
这应该这样做：
var query = dc.Regions.AsQueryable();
ParameterExpression pe = Expression.Parameter(typeof(Region),"region");
Expression id = Expression.PropertyOrField(pe,"RegionID");
Expression two = Expression.Constant(2);
Expression e1 = Expression.GreaterThan(id,two);
Expression description = Expression.PropertyOrField(pe,"RegionDescription");
MethodInfo method = typeof(string).GetMethod("Contains",new[] {typeof(string)});
Expression ern = Expression.Constant("ern",typeof(string));
Expression e2 = Expression.Call(description,method,ern);
Expression e3 = Expression.And(e1,e2);
MethodCallExpression whereCallExpression = Expression.Call(
typeof(Queryable),"Where",new Type[] { query.ElementType },query.Expression,Expression.Lambda<Func<Region,bool>>(e3,new ParameterExpression[] { pe }));
var results = query.Provider.CreateQuery<Region>(whereCallExpression);
List<Region> lst = results.ToList();
要从结果集中选择RegionID,请执行以下操作：
MethodCallExpression selectExpression = Expression.Call(
typeof(Queryable),"Select",new[]{ typeof(Region),typeof(int)},whereCallExpression,int>>(id,pe));
var regionIDsQuery = query.Provider.CreateQuery<int>(selectExpression);
List<int> regionIDs = regionIDsQuery.ToList();
