JSON令牌不断返回NULL

Question

我有一个JSON.NET令牌，如下所示：

{
  "version": "1.0",
  "createdBy": "PowerStats v1.0",
  "test": "ohoh",
  "DSNumber": {
    "value": "82"
  },
  "title": {
    "value": ""
  },
  "footnote": {
    "value": ""
  },
  "flagRSE": {
    "value": "30,50",
    "symbol": "!,!!"
  },
  "weight": {
    "type": "0",
    "varName": "WTA000",
    "label": "weight_var"
  },
  "filters": {
    "filter_1": {
      "component": {
        "varName": "JOBEARN2",
        "filterType": "Range",
        "format": "Cont",
        "label": "Job: Earnings from work while enrolled (including work-study)",
        "element": {
          "startValue": "1",
          "endValue": "",
          "label": "X >= 1"
        }
      }
    },
    "filter_2": {
      "component": {
        "varName": "JOBROLE2",
        "filterType": "Dist",
        "format": "Disc",
        "label": "Job: Primary role as student or employee (including work-study)",
        "element": {
          "value": "1",
          "label": "A student working to meet expenses"
      }
    }
  },
  "columns": {
    "component": {
      "tag": "CAT",
      "missingValue": "4,5,6,7,8,9,10,13,14,15,16,17,18,19,20,21,22,23,-3",
      "format": "Disc",
      "varName": "MAJORS23",
      "label": "Field of study: undergraduate (23 categories)",
      "element": [
        {
          "value": "0",
          "label": "Undecided"
        },
        {
          "value": "1",
          "label": "Computer and information sciences"
        },
        {
          "value": "2",
          "label": "Engineering and engineering technology"
        },
        {
          "value": "3",
          "label": "Biological and physical science, science tech"
        },
        {
          "value": "11",
          "label": "Personal and consumer services"
        },
        {
          "value": "12",
          "label": "Manufacturing,construction,repair & transportation"
        }
      ]
    }
  },
  "rows": {
    "component": [
      {
        "tag": "CUT",
        "missingValue": "",
        "format": "Cont",
        "varName": "GPA",
        "label": "Grade point average",
        "element": [
          {
            "startValue": "1",
            "endValue": "249",
            "label": "Lower than 2.50"
          },
          {
            "startValue": "250",
            "endValue": "299",
            "label": "2.50 - 2.99"
          },
          {
            "startValue": "300",
            "endValue": "349",
            "label": "3.00 - 3.49"
          },
          {
            "startValue": "350",
            "endValue": "400",
            "label": "3.50 or higher"
          }
        ]
      },
      {
        "tag": "CAT",
        "missingValue": "",
        "format": "Disc",
        "varName": "STEMMAJ",
        "label": "Major field of study with a focus on STEM fields",
        "element": [
          {
            "value": "1",
            "label": "Math/Computer/Sciences/Engineering/Technologies"
          },
          {
            "value": "2",
            "label": "Social/behavioral sciences"
          },
          {
            "value": "4",
            "label": "Non-STEM field"
          },
          {
            "value": "5",
            "label": "Undecided or not in a degree program"
          }
        ]
      }
    ]
  }
}

但当我这样做时：

// This is correct and returns 1.
int columnsCount = token["columns"].Count();
// This errors out with the error below
int componentCount = token["columns.component"].Count();

错误为：

"Value cannot be null. Parameter name: source" 

但为什么呢？

Answer 1

token["columns"]本身就是另一个JSON对象。您需要以相同的方式引用其中的内容，即

token["columns"]["component"]