var query=....;
if (!string.IsNullOrEmpty(a))  {  query = query.Where(p => p.name == a);  }

public static class PredicateBuilder{/// <summary>/// 构造函数使用True时：单个AND有效，多个AND有效；单个OR无效，多个OR无效；混合时写在AND后的OR有效/// 构造函数使用False时：单个AND无效，多个AND无效；单个OR有效，多个OR有效；混合时写在OR后面的AND有效/// </summary>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.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);}public static Predicate<T> ConvertToPredicate<T>(this Func<T, bool> func){return new Predicate<T>(func);}}

sql语句的想要实现的内容如下，重点关注where后面的条件部分：
select ... from ... where (Test.id = 3 and Test.name = "N3") or (Test.id = 4 and Test.name = "N4")
linq方式的动态实现如下：
var predicate = PredicateBuilder.False<Test>();
foreach (var keyword in searchKeywords)
{int nTemp = keyword.id;string sTemp = keyword.name;predicate = predicate.Or(p => p.id == nTemp && p.name == sTemp);
}IEnumerable<Test> results = lst.Where(predicate.Compile());

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();

var query=...;
var basequery=...;foreach(var seq in condictions)
{
query=query.union(basequery.where(a=>a.name.contains("xxx"));
}