构建通用搜索表达式- InvalidOperationException:与不能为空的类型一起使用的Coalesce

Question

我有一个问题，建立一个通用的搜索表达式，将工作在多个不同类型的对象。我遇到的问题是，一些属性将包含一个空值。

这个函数将对每个属性调用ToString方法，并查看它是否包含该特定属性的搜索值。问题是，如果它是空的。如果我尝试合并，它会抛出InvalidOperationException: Coalesce used with type that cannot be null异常。

下面是一个简单的类:

public class SomeObject
{
  public int IntValue { get; set; }
  public string StringValue { get; set; }
}

示例程序:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
class Program
{
  static void Main(string[] args)
  {
    //Generate data
    var list = (new List<SomeObject>
    {
      new SomeObject { IntValue = 5, StringValue = "abc" },
      new SomeObject { IntValue = 10, StringValue = "abc" },
      new SomeObject { IntValue = 10 },
      new SomeObject { StringValue = "xyz" },
      new SomeObject()
    }).AsQueryable<SomeObject>();
    var searchValues = new List<KeyValuePair<string, string>>
    {
      new KeyValuePair<string, string>("IntValue", "5"),
      new KeyValuePair<string, string>("StringValue", "abc")
    };
    var whereExp = GenerateSearchExpression<SomeObject>(searchValues);
    var asdf = list.Where(whereExp).ToList();
    System.Diagnostics.Debugger.Break();
  }
  static Expression<Func<SomeObject, bool>> GenerateSearchExpression<dataType>(List<KeyValuePair<string, string>> values)
  {      
    //Get the properties of the data type
    var objectType = typeof(dataType);
    var objProps = objectType.GetProperties();
    Expression whereExpr = Expression.Constant(true);
    var paramExpr = Expression.Parameter(typeof(SomeObject), "val");
    //Cycle through each property
    foreach (var searchValue in values)
    {
      var propExpr = Expression.Property(paramExpr, searchValue.Key);
      var emptyString = Expression.Constant("");
      //InvalidOperationException: Coalesce used with type that cannot be null
      var coalesceExpr = Expression.Coalesce(propExpr, emptyString);
      var toStringExpr = Expression.Call(coalesceExpr, "ToString", new Type[0]);
      var toLowerExpr = Expression.Call(toStringExpr, "ToLower", new Type[0]);
      var containExpr = Expression.Call(toLowerExpr, typeof(string).GetMethod("Contains"), Expression.Constant(searchValue.Value));
      //Add this to the exisiting where expression.
      whereExpr = Expression.And(whereExpr, containExpr);
    }
    var foobar = Expression.Lambda<Func<SomeObject, bool>>(whereExpr, paramExpr);
    return foobar;
  }
}

有什么办法吗?

Answer 1

我需要使用propExpr.Type.IsValueType属性:

这是最后一个函数:

  static Expression<Func<SomeObject, bool>> GenerateSearchExpression<dataType>(List<KeyValuePair<string, string>> values)
  {      
    //Get the properties of the data type
    var objectType = typeof(dataType);
    var objProps = objectType.GetProperties();
    Expression whereExpr = Expression.Constant(true);
    var paramExpr = Expression.Parameter(typeof(SomeObject), "val");
    MethodCallExpression toStringExpr;
    foreach (var searchValue in values)
    {
      var propExpr = Expression.Property(paramExpr, searchValue.Key);
      if (propExpr.Type.IsValueType == true)
      {
        toStringExpr = Expression.Call(propExpr, "ToString", new Type[0]);
      }
      else
      {
        var emptyString = Expression.Constant("");
        var coalesceExpr = Expression.Coalesce(propExpr, emptyString);
        toStringExpr = Expression.Call(coalesceExpr, "ToString", new Type[0]);            
      }
      var toLowerExpr = Expression.Call(toStringExpr, "ToLower", new Type[0]);
      var containExpr = Expression.Call(toLowerExpr, typeof(string).GetMethod("Contains"), Expression.Constant(searchValue.Value));         
      //Add this to the exisiting where expression.
      whereExpr = Expression.And(whereExpr, containExpr);
    }
    var foobar = Expression.Lambda<Func<SomeObject, bool>>(whereExpr, paramExpr);
    return foobar;
  }