Lambda动态条件查询

using System;
using System.Linq;
using System.Linq.Expressions;
using System.Collections.Generic;

public static class PredicateBuilder
{
public static Expression<Func<T, bool>> True<T>() { return f => true; }
public static Expression<Func<T, bool>> False<T>() { return f => false; }

public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1,
Expression<Func<T, bool>> expr2)
{
var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
return Expression.Lambda<Func<T, bool>>
(Expression.OrElse(expr1.Body, invokedExpr), expr1.Parameters);
}

public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1,
Expression<Func<T, bool>> expr2)
{
var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
return Expression.Lambda<Func<T, bool>>
(Expression.AndAlso(expr1.Body, invokedExpr), expr1.Parameters);
}

public IList<Memo> GetMemoList(string department, bool includePublic)
{
var filterExpression = PredicateBuilder.True<Memo>();
filterExpression = filterExpression.And(x => x.ID > 0);

if (department != "")
{
filterExpression = filterExpression.And(x => x.Department == department);
}

if (includePublic)
{
var orExpression = PredicateBuilder.False<Memo>();
orExpression = orExpression.Or(x => x.IsPublic == true);
filterExpression = filterExpression.Or(orExpression);
}

return db.Memo.Where(filterExpression.Compile())
.OrderByDescending(x => x.IsTop)
.ThenByDescending(x => x.ID).ToList();
}