'System.Nullable`1 [T]'上的GenericArguments [0]，'MvcApplication66.Controllers.HomeController + Info'違反了類型參數'T'的約束條件

Question

我在家庭控制器中的數據表如下：

public DataTable GetTable() 
    { 
     DataTable table = new DataTable(); 
     table.Columns.Add("Dosage", typeof(int)); 
     table.Columns.Add("Drug", typeof(string)); 
     table.Columns.Add("Patient", typeof(Info)); 
     table.Columns.Add("Date", typeof(DateTime)); 

     table.Rows.Add(25, "Indocin", new Info("India"), DateTime.Now); 
     table.Rows.Add(50, "Enebrel", new Info("UK"), DateTime.Now); 
     table.Rows.Add(10, "Hydralazine", new Info("Bhutan"), DateTime.Now); 
     table.Rows.Add(21, "Combivent", new Info("India"), DateTime.Now); 
     table.Rows.Add(100, "Dilantin", new Info("GreenLand"), DateTime.Now); 
     return table; 
    } 

信息類如下

public class Info 
    { 
     public string Address { get; set; } 
     public Info(string Add) { 
      this.Address = Add; 
     } 
    } 

然後我調用TableSerialization方法如下

public ActionResult Index(){ 
     DataTable table = GetTable(); 
     ViewBag.dataSource = table; 
     DataTableOperations dp = new DataTableOperations(); 
     dp.DataTableSerialize(table.AsEnumerable(), li); 
     return View(); 
    } 

在大taTable序列化我已經定義如下

 public static Dictionary<string, Type> DataTableSerialize(this IQueryable datasource) 
     { 
      var DataColumns = datasource.Take(1).Cast<DataRow>().CopyToDataTable().Columns.Cast<DataColumn>(); 
      var type = typeof(Nullable<>); 
      var cols = DataColumns.Select(column => new { column = column.ColumnName, ColumnType = column.DataType, IsNullable = column.AllowDBNull }).ToList(); 
      return cols.ToDictionary(d => d.column, d => d.IsNullable && (d.ColumnType != typeof(string)) ? type.MakeGenericType(new[] { d.ColumnType }) : d.ColumnType); 
     } 

我在

（d.ColumnType！= typeof運算（字符串））得到了錯誤？ type.MakeGenericType（new [] {d.ColumnType}）：d.ColumnType）;

我的堆棧跟蹤

[TypeLoadException: GenericArguments[0], 'MvcApplication66.Controllers.HomeController+Info', on 'System.Nullable`1[T]' violates the constraint of type parameter 'T'.] 
    System.RuntimeTypeHandle.Instantiate(RuntimeTypeHandle handle, IntPtr* pInst, Int32 numGenericArgs, ObjectHandleOnStack type) +0 
    System.RuntimeTypeHandle.Instantiate(Type[] inst) +94 
    System.RuntimeType.MakeGenericType(Type[] instantiation) +214 

[ArgumentException: GenericArguments[0], 'MvcApplication66.Controllers.HomeController+Info', on 'System.Nullable`1[T]' violates the constraint of type 'T'.] 
    System.RuntimeType.ValidateGenericArguments(MemberInfo definition, RuntimeType[] genericArguments, Exception e) +4366838 
    System.RuntimeType.MakeGenericType(Type[] instantiation) +230 

其中i犯的錯誤

Answer 1

Nullable泛型類型不能與類或字符串一起使用，因此MakeGenericType在複雜字段（如Info類）的情況下會給出錯誤。

，所以我改變的條件，具體如下：

d => (d.IsNullable && (d.ColumnType != typeof(string)) && (d.ColumnType.IsValueType)) ? type.MakeGenericType(new[] 
{ d.ColumnType }) : d.ColumnType); 

Answer 2

Nullable<>的type參數被約束爲一個非空值類型。你沒有給它一個不可爲空的值類型。要麼不要構造一個空值，要麼當你這樣做時不要將它傳遞給一個引用類型。