拉姆达表达式 追加 条件判断 Expression<Func<T, bool>>

public static class PredicateBuilder
{

/// <summary>
/// 机关函数应用True时：单个AND有效，多个AND有效；单个OR无效，多个OR无效；混应时写在AND后的OR有效
/// </summary>
/// <typeparam name="T"></typeparam>
/// <returns></returns>
public static Expression<Func<T, bool>> True<T>() { return f => true; }

/// <summary>
/// 机关函数应用False时：单个AND无效，多个AND无效；单个OR有效，多个OR有效；混应时写在OR后面的AND有效
/// </summary>
/// <typeparam name="T"></typeparam>
/// <returns></returns>
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.Or(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.And(expr1.Body, invokedExpr), expr1.Parameters);
}
}

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var where = PredicateBuilder.True<SYS_Message>();
where = where.And(p => p.isActive == true);

if (receiveUserId!=null)
{
where = where.And(p => p.MessageReceiveUser == receiveUserId);
}
if (!string.IsNullOrEmpty(messageMain))
{
where = where.And(p =>messageMain.Contains( p.MessageMain) );
}
if ( !string.IsNullOrEmpty(receivrUserName))
{
where = where.And(p =>messageMain.Contains( p.MessageMain)&&aliexpressEntities.YH_User.Where(U=>U.isActive==true&& receivrUserName.Contains(U.first_name)).Select(U=>U.userId).ToList().Contains(Convert.ToInt32( p.MessageReceiveUser)) );

}
if (startTime!=null)
{
where = where.And(p =>p.createTime>=startTime );
}
if (endTime != null)
{
where = where.And(p => p.createTime <= endTime);
}
EntityList = aliexpressEntities.SYS_Message.Where(where.Compile()).OrderByDescending(p => p.createTime).Skip(((int)pageIndex - 1) * pageSize).Take(pageSize).ToList();
protectCount = aliexpressEntities.SYS_Message.Where(where.Compile()).Count();
protectPage = Convert.ToInt32(Math.Ceiling(Convert.ToDouble(protectCount) / pageSize));