c#的executescalar用法有哪几种

C#中的ExecuteScalar方法用于履行查询并返回结果集中的第一行的第一列的值。以下是ExecuteScalar方法的常见用法：
1. 履行一个简单的SELECT查询，并获得返回的单个值。
```csharp
string queryString = "SELECT COUNT(*) FROM Customers";
using (SqlConnection connection = new SqlConnection(connectionString))
{
SqlCommand command = new SqlCommand(queryString, connection);
connection.Open();
int count = (int)command.ExecuteScalar();
Console.WriteLine("Total number of customers: " + count);
}
```
2. 使用参数化查询履行查询，并获得返回的单个值。
```csharp
string queryString = "SELECT FirstName FROM Customers WHERE LastName = @LastName";
using (SqlConnection connection = new SqlConnection(connectionString))
{
SqlCommand command = new SqlCommand(queryString, connection);
command.Parameters.AddWithValue("@LastName", "Smith");
connection.Open();
string firstName = (string)command.ExecuteScalar();
Console.WriteLine("First name of the customer: " + firstName);
}
```
3. 履行存储进程并获得返回的单个值。
```csharp
using (SqlConnection connection = new SqlConnection(connectionString))
{
SqlCommand command = new SqlCommand("GetTotalSales", connection);
command.CommandType = CommandType.StoredProcedure;
connection.Open();
decimal totalSales = (decimal)command.ExecuteScalar();
Console.WriteLine("Total sales: " + totalSales);
}
```
需要注意的是，在使用ExecuteScalar方法时，需要根据查询返回的数据类型来进行强迫类型转换。在处理返回值之前，还应当进行必要的毛病检查和异常处理。