如何使用DBNull.Value检查表列是否为空，如果不为空则返回默认值

using System;
using System.Data.SqlClient;

// 假设数据库连接字符串为 connectionString
string connectionString = "your_connection_string";

// 假设要查询的表名为 tableName
string tableName = "your_table_name";

// 假设要查询的列名为 columnName
string columnName = "your_column_name";

// 假设默认值为 defaultValue
string defaultValue = "your_default_value";

try
{
    using (SqlConnection connection = new SqlConnection(connectionString))
    {
        connection.Open();

        // 构造 SQL 查询语句
        string query = $"SELECT {columnName} FROM {tableName}";

        using (SqlCommand command = new SqlCommand(query, connection))
        {
            using (SqlDataReader reader = command.ExecuteReader())
            {
                if (reader.Read())
                {
                    // 获取表列值
                    object columnValue = reader.GetValue(0);

                    // 检查表列是否为空
                    if (columnValue != DBNull.Value)
                    {
                        // 表列不为空，返回表列值
                        Console.WriteLine($"The column value is: {columnValue}");
                    }
                    else
                    {
                        // 表列为空，返回默认值
                        Console.WriteLine($"The column value is null. Returning default value: {defaultValue}");
                    }
                }
                else
                {
                    Console.WriteLine("No rows found.");
                }
            }
        }
    }
}
catch (Exception ex)
{
    Console.WriteLine($"An error occurred: {ex.Message}");
}