How to handle the type mismatch during model binding in NET 5?

"type mismatch during model binding"

Firstly, Price value here is a string:

"Price": "28.78",

Yet you define you proerty in your class as a decimal type.

Yes you have a "type mismatch" error. The error message is very explicit:

Message=The JSON value could not be converted to System.Nullable`1[System.Decimal]. Path: $.Config.Price | LineNumber: 3 | BytePositionInLine: 24.

You will need to write a custom converter to convert between value types. The alternative, if possible, is to correctly serialize the value as a number. This is the prefered option if possible. It should look like this:

"Price": 28.78,

Here is how to write a custom converter for the System.Text.Json namespace. Newtownsoft Custom Converters are a little bit different - you can learn more here: Working with JSON in C# & VB[^]

public class StringToDecimalConverter : JsonConverter<decimal>
{
    public override decimal Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        decimal.TryParse(reader.GetString()!, out decimal value);
        return value;
    }

    public override void Write(Utf8JsonWriter writer, decimal value, JsonSerializerOptions options)
    {
        throw new NotImplementedException();
    }
}

There are two ways that you can use the Custom Converter:
1. JsonSerializerOptions
2. As a Property Atteribute.

You can read more about Custom Converters here: How to write custom converters for JSON serialization - .NET | Microsoft Docs[^] - there are how-to examples.

Here we will use Property Attributes:

public class Config
{
    public string? State { get; set; }

    [JsonConverter(typeof(StringToDecimalConverter))]  
    public decimal? Price { get; set; }

    [JsonConverter(typeof(StringToDecimalConverter))]  
    public decimal? discount { get; set; }

    public Order? Order { get; set; }
}

Now the Price and discount properties will not throw errors.

The next issue that you have is your saletype property. This will throw a similar error as you cannot convert String to Enum. Luckily, there is a pre-built JsonStringEnumConverter class. You can read more here: https://docs.microsoft.com/en-us/dotnet/api/system.text.json.serialization.jsonstringenumconverter?view=net-6.0[^]. This is staight forward to use:

public enum SaleType
{
    COD,
    ONLINE,
    PICKUP,
    OTHER
}

public class Order
{
    [JsonConverter(typeof(JsonStringEnumConverter))]  
    public SaleType? saletype { get; set; }
}

The last issue that you have is your Order.itemid property. You can not convert a string to an int. I have written the StringToDecimalConverter converter for you, StringToIntegerConverter I will leave to you.

** UPDATE **

For Completeness, here is my test code:

1. Program.cs (dot net 6.0)

using System.Diagnostics;
using System.Text.Json;

string json =
    @"{
    ""Config"": {
        ""State"": ""combodia"",
        ""Price"": ""28.78"",
        ""discount"": """",
        ""Order"": {
            ""saletype"": ""PICKUP""
        }
    }
}";

var data = JsonSerializer.Deserialize<Root>(json);

Debugger.Break();

2. Classes used:

using System.Text.Json.Serialization;

public enum SaleType
{
    COD,
    ONLINE,
    PICKUP,
    OTHER
}

public class Order
{
    [JsonConverter(typeof(JsonStringEnumConverter))]  
    public SaleType? saletype { get; set; }
    
    //public int? itemid { get; set; }
}

public class Config
{
    public string? State { get; set; }

    [JsonConverter(typeof(StringToDecimalConverter))]  
    public decimal? Price { get; set; }

    [JsonConverter(typeof(StringToDecimalConverter))]  
    public decimal? discount { get; set; }

    public Order? Order { get; set; }
}

public class Root
{
    public Config? Config { get; set; }
}

public class StringToDecimalConverter : JsonConverter<decimal>
{
    public override decimal Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        decimal.TryParse(reader.GetString()!, out decimal value);
        return value;
    }

    public override void Write(Utf8JsonWriter writer, decimal value, JsonSerializerOptions options)
    {
        throw new NotImplementedException();
    }
}

Code runs and deserializes successfully.