linq 实现查询字符串拼接 : And 和 OR 两种方式

N年前我们是这样来 拼接查询字符串的:

[c-sharp]
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public string Test(string a, string b, string c,string d)
{
string sql = "SELECT * FROM Users WHERE 1=1";
if (!string.IsNullOrEmpty(a))
{
sql += " AND name='" + a + "'";
}
if (!string.IsNullOrEmpty(b))
{
sql += " AND age='" + b+ "'";
}
if (!string.IsNullOrEmpty(c))
{
sql += " AND sex='" + c + "'";
}
if (!string.IsNullOrEmpty(d))
{
sql += " AND address='" + d + "'";
}
return sql.ToString();
}

现在我们使用linq来实现上边的代码：

[c-sharp]
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public void Test(string a, string b, string c,string d)
{
QueryContext query = new QueryContext();
var q = from u in query.Users
select u;
if (!string.IsNullOrEmpty(a))
{
q = q.Where(p => p.name == a);
}
if (!string.IsNullOrEmpty(b))
{
q = q.Where(p => p.age == b);
}
if (!string.IsNullOrEmpty(c))
{
q = q.Where(p => p.sex == c);
}
if (!string.IsNullOrEmpty(d))
{
q = q.Where(p => p.address == d);
}
q.ToList(); //上边的所有if，只有到此处才会执行
}

ps: 如果查询字符串内 不是AND，而是OR 的话

如：

[c-sharp]
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public string Test(string a, string b, string c,string d)
{
string sql = "SELECT * FROM Users WHERE 1=1";
if (!string.IsNullOrEmpty(a))
{
sql += " OR name='" + a + "'";
}
if (!string.IsNullOrEmpty(b))
{
sql += " OR age='" + b+ "'";
}
if (!string.IsNullOrEmpty(c))
{
sql += " OR sex='" + c + "'";
}
if (!string.IsNullOrEmpty(d))
{
sql += " OR address='" + d + "'";
}
return sql.ToString();
}

此种情况，通过linq可以这样实现：

[c-sharp]
view plaincopy

public void Test(string a,string b,string c,string d)
{
QueryContext query = new QueryContext();
var q1 = from u in query.Users
where u.name== a && a != ""
|| u.age == b && b != ""
|| u.sex == c && c != ""
|| u.address ==d && d !=""
select u;
q1.ToList();
}