使用 LINQ 将 SQL 表转换为 CSV

Question

我正在尝试将表格放入逗号分隔的 CSV 文件中。我目前正在实现这样的目标：

var csv = new StringBuilder("id,Name,Address,City,State\n");
var delimiter = ",";
var newline = "\n";

using (var db = MyDatabase())
{
     var query = from c in db.Customers select c;
     foreach(var q in query)
         csv.Append(q.id).Append(delimiter)
             .Append(q.name).Append(delimiter)
             .Append(q.address).Append(delimiter)
             .Append(q.city).Append(delimiter)
             .Append(q.state).Append(newline);
}

这会创建一个 csv 罚款。但我的问题是，是否有一种更简化的方法可以实现此无需单独指定每一列？在某些情况下，我想做一个“全选”，但由于表格有很多列，这看起来有点笨拙。

Answer 1

List<Customer> query = from c in db.Customers select c;
        foreach (var q in query)
        {
            Type type = typeof(Customer);
            PropertyInfo[] properties = type.GetProperties();
            foreach (PropertyInfo property in properties)
            {
                string value = property.GetValue(type, null) != null
                    ? property.GetValue(type, null).ToString() 
                    : string.Empty; // or whatever you want the default text to be
                csv.Append(q.id).Append(delimiter);
            }
        }

这将循环查询查询中的每个 Customer，然后循环遍历 Customer 中的每个属性并写入值。这是假设您希望将客户的每个属性写入 CSV 文件。这至少应该给你关于如何做你想要完成的事情的想法。

Answer 2

var props = typeof(Customer).GetProperties(
          BindingFlags.Instance|BindingFlags.Public
      );

StringBuilder result = new StringBuilder(
         (
             from prop in props select prop.Name
         ).Join(',') + "\n"
     );

Function<string,Customer> project = ( 
   Customer c => (
           from prop in props 
           select prop.GetValue(c).ToString()
       ).Join(",")+"\n";
)

using (var db = MyDatabase())
{
    var query = from c in db.Customers select project(c);
    foreach (var res in query) {
        result.append(res);
    }
}

您可以轻松地使其更加通用，但随后您将需要缓存反射的 PropertyInfo 以避免在类型上大量使用反射。

警告

：您应该正确转义属性值，因为换行符、逗号、引号等可能会破坏您的 CSV 文件。更好的是，使用已经解决的库 写入正确的 CSV 文件的问题。

Answer 3

string delimiter = "|";
string newLine = Environment.NewLine;

using (mCustDataContext = new CustDataContext()) {
    var props = typeof(Customer).GetProperties();

    StringBuilder headers = new StringBuilder(string.Join(delimiter, props.Select(c => c.Name).ToArray()) + newLine);

    Func<Customer, string> myFunc = c => 
         string.Join(delimiter, props.Select(b => b.GetValue(c,null))) + newLine;

    var query = mCustDataContext.Customers.Select(myFunc);

    foreach(var q in query)
        headers.Append(q);

    File.WriteAllText(yourFileName, headers.ToString(), mEncoding);
}

Answer 4

await conn.OpenAsync(token).ConfigureAwait(false);

const string Sql = "SELECT * FROM table_name;";

DataTable dataTable = new DataTable();        
using var command = new SqlCommand(Sql, conn);        
using var reader = await command.ExecuteReaderAsync(token);        
dataTable.Load(reader);


var columns = dataTable.Columns.Cast<DataColumn>()
    .Select(c => c.ColumnName);        
var results = dataTable.Rows.Cast<DataRow>()
    .Select(row => columns.ToDictionary(column => column, column => row[column]));        
var head_str = string.Join(";", columns.Values)));        
var result_str = string.Join("\r\n", results.Select(t => string.Join(";", t.Values)));