linq 实现查询字符串拼接 : And 和 OR 两种方式

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();
}

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
}

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();
}

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();
}