CS0121 – The call is ambiguous between the following methods

The CS0121 error in C# occurs when the compiler cannot determine which overloaded method to call because multiple methods match the provided arguments. This error typically happens due to ambiguous method overloads, implicit type conversions, or conflicting parameter types. Here’s a detailed guide to understanding, troubleshooting, and resolving this error:

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1. Common Causes

1. Ambiguous Method Overloads:

• Multiple method overloads have parameters that can accept the provided arguments.

1. Implicit Type Conversions:

• Implicit type conversions make it unclear which overload to call.

1. Conflicting Parameter Types:

• The provided arguments match multiple overloads due to similar parameter types (e.g., int and long).

1. Default Parameter Values:

• Default parameter values make it unclear which overload to call.

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2. Troubleshooting Steps

Check Method Overloads

1. Review Method Definitions:

• Identify all overloads of the method and their parameter types.
• Example:
  csharp void MyMethod(int x) { } void MyMethod(long x) { }

1. Check Argument Types:

• Verify the types of the arguments being passed to the method.

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Check Implicit Type Conversions

1. Identify Implicit Conversions:

• Check if implicit type conversions are causing ambiguity:
  csharp MyMethod(10); // Could call MyMethod(int) or MyMethod(long)

1. Use Explicit Casts:

• Use explicit casts to resolve ambiguity:
  csharp MyMethod((int)10); // Calls MyMethod(int) MyMethod((long)10); // Calls MyMethod(long)

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Check Default Parameter Values

1. Review Default Values:

• Check if default parameter values are causing ambiguity: void MyMethod(int x, int y = 0) { } void MyMethod(int x, string y = "") { } MyMethod(10); // Ambiguous: Could call either overload

1. Specify Default Values Explicitly:

• Specify default values explicitly to resolve ambiguity:
  csharp MyMethod(10, 0); // Calls the first overload MyMethod(10, ""); // Calls the second overload

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Check Conflicting Parameter Types

1. Identify Conflicting Types:

• Check if similar parameter types (e.g., int and long) are causing ambiguity.

1. Use Explicit Types:

• Use explicit types or casts to resolve ambiguity:
  csharp MyMethod(10L); // Calls MyMethod(long)

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3. Resolving the Error

For Ambiguous Method Overloads

1. Remove or Refactor Overloads:

• Remove or refactor ambiguous overloads to avoid conflicts.

1. Use Explicit Arguments:

• Provide explicit arguments to resolve ambiguity:
  csharp MyMethod(10, 0); // Calls the first overload

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For Implicit Type Conversions

1. Use Explicit Casts:

• Use explicit casts to specify the intended type:
  csharp MyMethod((int)10); // Calls MyMethod(int)

1. Avoid Implicit Conversions:

• Avoid relying on implicit conversions in method calls.

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For Default Parameter Values

1. Specify Default Values:

• Specify default values explicitly to resolve ambiguity:
  csharp MyMethod(10, 0); // Calls the first overload

1. Remove Default Values:

• Remove default values from ambiguous overloads.

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For Conflicting Parameter Types

1. Use Explicit Types:

• Use explicit types or casts to resolve ambiguity:
  csharp MyMethod(10L); // Calls MyMethod(long)

1. Refactor Overloads:

• Refactor overloads to avoid similar parameter types.

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4. Preventing the Error

1. Avoid Ambiguous Overloads:

• Avoid creating method overloads with similar parameter types.

1. Use Strongly-Typed Code:

• Use strongly-typed code to avoid implicit conversions.

1. Regular Code Reviews:

• Conduct regular code reviews to identify and resolve ambiguity.

1. Enable Treat Warnings as Errors:

• Treat warnings as errors to enforce clean code practices:
  xml <PropertyGroup> <TreatWarningsAsErrors>true</TreatWarningsAsErrors> </PropertyGroup>

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