此表达式会导致副作用，不会被评估

Question

我在 LINQ 查询中收到此错误，我不确定原因。当我运行 table 是空的但是当我添加一个断点强制结果然后它工作找到并完美地添加所有数据

    private void GetSQLOraclelinqData()
    {

        var TstarData = GetTrackstarTruckData();
        var M5Data = GetM5Data();

        DataTable ComTable = new DataTable();

        foreach (DataColumn OraColumn in M5Data.Columns)
        {
            ComTable.Columns.Add(OraColumn.ColumnName, OraColumn.DataType); 

        }
        foreach (DataColumn SQLColumn in TstarData.Columns)
        {
            if (SQLColumn.ColumnName == "VehicleName")
                ComTable.Columns.Add(SQLColumn.ColumnName + 2, SQLColumn.DataType);
            else
                ComTable.Columns.Add(SQLColumn.ColumnName, SQLColumn.DataType);

        }

        var results = TstarData.AsEnumerable().Join(M5Data.AsEnumerable(),
            a => a.Field<String>("VehicleName"),
            b => b.Field<String>("Unit_NO"),
                (a, b) =>
                {

                    DataRow row = ComTable.NewRow();
                    row.ItemArray = a.ItemArray.Concat(b.ItemArray).ToArray();
                    ComTable.Rows.Add(row);
                    return row;

                });

        SQLDataTable.ItemsSource = ComTable.DefaultView;
    }

Answer 1

像这样添加 ToList：

var results = TstarData.AsEnumerable().Join(M5Data.AsEnumerable(),
            a => a.Field<String>("VehicleName"),
            b => b.Field<String>("Unit_NO"),
                (a, b) =>
                {

                    DataRow row = ComTable.NewRow();
                    row.ItemArray = a.ItemArray.Concat(b.ItemArray).ToArray();
                    ComTable.Rows.Add(row);
                    return row;

                }).ToList();

此表达式会导致副作用，不会被评估

This Expression causes side effect and will not be evaluated

c#

linq

debugging